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DIY Auto Service: AC System Diagnosis by Symptom

Michael Thomas, automotive and diesel technician, is the author of the McGraw-Hill textbook "Truck and Trailer Systems."

AC System Diagnosis

The typical complaint for an AC problem is the AC blows warm air. This condition has a few possible causes: the AC system could be low on refrigerant, an electrical problem may not be allowing the compressor to turn on, there could be an internal problem in the system, or perhaps there is a problem with the heater/AC controls. Since the most common problem is usually the refrigerant, we will start there.

Three-Part Series

This series is divided into three parts. The AC system operation, AC system service, and the diagnosis of a malfunctioning system. To be able to service and repair an AC system, read through all three articles.

DIY Auto Service: Air Conditioning (AC) System Operation with TXV or Orifice Tube

DIY Auto Service; AC System Service and Component Replacement

DIY Auto Service; AC System Diagnosis by Symptom

Static AC Gauge Pressures

Static Pressure means the system is not running. Compare this pressure with the ambient temperature to gauge the amount of refrigerant in the system. Less than the chart indicate low refrigerant charge and higher may indicated an overcharge.

Static Pressure means the system is not running. Compare this pressure with the ambient temperature to gauge the amount of refrigerant in the system. Less than the chart indicate low refrigerant charge and higher may indicated an overcharge.

Static Pressures

There is a relationship between the pressure in the system and temperature. A temperature pressure chart can be used to compare the pressure of the refrigerant at a given temperature. Since there is no way to know exactly how much refrigerant is in the system, without removing it and measuring it, we will use the pressure and compare it to a pressure temperature chart.

  • Hook up a gauge set or recovery machine.
  • Record the pressures. High and Low Side should be equal.
  • Measure the ambient temperature.
  • Does the pressure match the temperature pressure chart?

    1. If the pressure matches this means the refrigerant level is close.
    2. If the pressure is lower than the chart, the refrigerant level is low. The system is probably leaking.
    3. If the pressure is zero, that is atmospheric pressure and means the system has a big leak.
    4. If the pressure is higher than the chart, it is either overcharged or may have air (non-condensable) in the system.

Leak Testers and Refrigerant Identifiers

If the Static Pressure is low, a leak is probably present. Use the leak tester for larger leaks and the Dye with a black light for smaller leaks. Identify the refrigerant before hooking up to a recovery machine.

If the Static Pressure is low, a leak is probably present. Use the leak tester for larger leaks and the Dye with a black light for smaller leaks. Identify the refrigerant before hooking up to a recovery machine.

Running System Pressures

Start the engine and operate the AC system. Perform a visual check to see if the compressor is running.

  1. Was a click heard when the AC was engaged? Is the center hub turning?
  2. Observe the pressure gauges. Does the low side pressure start going down and the high side going up?
  3. After the system runs for awhile, raise the RPMs up to about 1,500 RPMs.
  4. The system should be about 25 to 30 psi on the low side and 200 to 250 psi on the high side. If the ambient temperature is higher or lower than normal (70 to 80°F) the pressures will go up or down accordingly.

Low and High Side Pressure the Same

No change in low side and high side pressures, the pressures are still equal.

  • Double check to see if the compressor clutch is engaged. If the center hub is turning, the compressor is not pumping.
  • Shut the engine off and rotate the center hub by hand. Is there any resistance?

The compressor may be faulty.

Higher Pressure on the Gauges

Higher Pressure on both the High and Low side may indicate an overcharge or lack of cooling at the Condenser.

Higher Pressure on both the High and Low side may indicate an overcharge or lack of cooling at the Condenser.

Higher-Than-Normal Pressures

Higher than normal pressures on both high and low side with the correct amount of refrigerant, could mean a problem with air flow thru the condenser.

  • Is the condenser or cooling fan working?
  • Does the condenser have bent or dirty fins?
  • Is there blockage inside the condenser?

Lower High and Low Pressures

Lower pressures may indicate low refrigerant level or a weak compressor.

Lower pressures may indicate low refrigerant level or a weak compressor.

Lower Low and High Side Pressures

Lower than normal pressures on both high and low side, with the correct amount of refrigerant, could mean a problem building pressure in the system or too much heat is being removed at the condenser. Is the ambient temperature low?

With the system running place a fender cover in front of the condenser and observe the pressure.

  • Does the pressure go up? Can the compressor build the pressure? If not the compressor may be worn out.
  • Is the cooling fan running constantly? A pressure switch or sensor could be bad.

Lower Low Side and Higher High Side Pressures

This condition usually means there is a restriction in the system or the TXV/Orifice Tube is blocked or closed. Feel the lines, is there a pressure/temperature drop at the TXV/Orifice Tube?

  • With a drop, there may be a restriction at the TXV or Orifice Tube.
  • With no drop, there may be a restriction upstream from the TXV or Orifice Tube. Follow the liquid line back towards the condenser to see if you can feel a pressure/temperature drop.

Higher Low Side and Lower High Side Pressures

Too much refrigerant is flowing into the evaporator.

  • The TXV is stuck open or the thermal bulb is not sensing the temperature correctly.
  • The orifice tube is too large of an opening or the o-ring sealing the outside of the orifice tube is not sealing.

System Pressures Are Normal, but Warm Air Is Blowing Out of Vents

Some times the refrigerant part of the system is not where the problem lies. The AC and Heater operate out of the same box or plenum typically under the dash. Doors control the direction of the airflow created by the electric blower motor. Depending on the system, there are at least three to four doors controlled by cables, vacuum actuators or electronic motors.

These doors are:

  • Hot/Cold temperature door - controls the amount of airflow through the heater core. During AC (Cold) operation, this door may force the air to bypass the heater core. When the temperature lever is set to Hot, all the air is forced through the heater core to use the heat from the engine coolant to heat the cab.
  • Fresh Air/Recirculate door - This door selects whether the air is drawn from the outside or circulate the air inside the vehicle. Recirculating the cooler drier air inside the passenger compartment can help maintain a cooler temperature instead of cooling the hot moist outside air. Typically some outside air is always added to keep the passenger compartment pressurized to prevent exhaust from entering the passenger compartment.
  • Floor/Vent/Defrost door or doors - This door or a combination of doors controls where the cooled or heated air is sent. Floor mode typically for heater operation since heat rises. Vent out the center of the dash for AC operation. Top of the dash to defrost the windshield during cold weather. There may also be combinations of floor/vent or floor/defrost to accommodate different situations.

AC Heater Plenum

The AC Heater box or plenum is located under the dash. Electronic actuators with position sensors are very popular to move the doors to direct the airflow and control temperature.

The AC Heater box or plenum is located under the dash. Electronic actuators with position sensors are very popular to move the doors to direct the airflow and control temperature.

With or Without Hot Water Valve Systems

There are two basic designs for the airflow:

  1. All the airflow goes thru both the evaporator then the heater core. A hot water valve closes the flow of hot coolant thru the heater core during the AC operation. If this valve doesn’t close, the AC cold air will be heated by the heater core. If the valve sticks closed, there will be no heat. The result is hot air out the vents. In the defrost mode, the air will be dehydrated by the evaporator then heated by the heater core for hot dry air on the windshield.
  2. The airflow goes thru the evaporator and a door directs the air either thru the heater core or around the heater core directed by the HOT to COLD selector. During AC COLD operation, the “Blend Door” routes the cold air around the heater core. During Heat operation, all the air passes thru the Evaporator and the Heater Core. When the temperature is set in between, part of the air will pass thru the Heater Core. No hot water valve is needed because in cool mode the door closes off airflow thru the heater core.

When the Air passes thru the evaporator and heater core as in example 1 above, a hot water valve is used to shut off the HOT water to the heater core. These Hot Water Valves can be; Cable operated, Air Operated and Electric Solenoid operated.

Door Controls and Actuators

To control the airflow thru the heater box or plenum a series of doors are used. To control these doors a variety of actuators have been used such as:

  • Manual cable operated doors have been around a long time. Cables connect levers in the dash control to the doors. When a lever is moved back and forth, like the hot/cold lever, it moves the door from one position to another. Typically moving the lever quickly from one extreme to the other results in a thumping sound as the door hits the stops. The Hot/Cold door would control airflow around or thru the heater control. Cables popping off or improper adjustments are typically what will go wrong with this style of control.
  • Vacuum operated actuators, used on cars and light duty trucks use (gas) engine vacuum to act upon a diaphragm which is connected to the doors. As the controls are moved, a hissing sound is heard. Vacuum leaks are the biggest problem with this type of control. If the vacuum supply is disconnected, the controls usually default to putting air on the windshield. Diesel engine light trucks using vacuum controlled systems use an electric or belt driven vacuum pump to supply the vacuum.
  • Pneumatic actuators operate similar to the vacuum type, except the trucks air system pressure is used to move the actuator diaphragms. Hissing under the dash could be from a leaking actuator or connection.
  • Electronic actuators use an electric motor to position the doors. These electric motors usually have a sensor built in to allow the system to know the location of the doors. When the controls are moved, a small electric motor buzz can usually be heard.

Service Tip: A common problem with all the actuator types are paper clips, pens, keys and other foreign objects falling down into the heater box from the dash defroster vents and jamming the doors. This could also strip out the door connection to the actuator rod.

Cabin Filter

Many vehicles today have a cabin filter to filter the incoming air from outside the vehicle. The filter looks like a small air filter from a car. Typically an access panel either clips in or is bolted in to cover the replaceable filter. The filter may be located under the dash or on the heater box in the engine compartment. This filter is often overlooked until it is entirely plugged up and causes very little airflow into the cab.

If the cabin filter is dirty the result will be low airflow. This may cause the AC or heater output to be reduced. Many times this is confused with poor AC or Heater performance due to the fact that the low airflow cannot keep the vehicle cool/warm.

Dirty Evaporator

The dirt on this evaporator has restricted the airflow and caused a lack of cooling.

The dirt on this evaporator has restricted the airflow and caused a lack of cooling.

Dirty Evaporator

The airflow across the evaporator is used for cooling the air. If dirt, fuzz, leaves or plastic bags are covering the surface, the airflow will be reduced along with the cooling capacity. The AC system may be in good working order but the air cannot have the heat removed as it should. The complaint will be lack of cooling the same condition as a plugged cabin filter.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: How about if the static pressure is not equal. 50 psi on low pressure and 180 psi on high pressure with an outside of temperature of 75F, what does this mean for diagnostics of an AC system?

Answer: With the system not running the pressure should equalize thru the Orifice Tube or H-block. When you shut off the AC you hear a hissing noise. I would say there is something wrong with the gauges. What happens when the system runs, do the gauges both move?

Question: How do you tell if the condenser is restricted ?

Answer: By the temperature in and out. Hot in, cold out is restricted. Usually, the high side is really high and low side really low.

Question: I have a 2008 Rav4. Twice I have tried to charge the A.C. system to the proper pressure. When the proper pressure is reached, the vents blow warm air and the compressor drops out. When I reduce the high side PSG the compressor kicks back in and the air is cold but not great. Could it be the hi pressure switch?

Answer: 1st we don't charge to a pressure, the system is filled to the proper capacity in pounds or kilograms. The system must be evacuated before charging.

Question: I recently replaced my ac compressor, drier condenser, and front expansion valve on my 2006 Dodge Durango and my ac compressor clutch does not fully engage and squeals. What should be my next step? 2006 dodge Durango slt 4.7l with front and rear ac.

Answer: 3 things to check

1. Clutch plate air gap between the plate and the pulley in specs.

2. Voltage at the compressor, should be 12V

3.Voltage drops in + or - side of the AC circuit, bad connections or corrosion (in the bottom of fuse panel) causing voltage drops.

Question: During a pressure test, the low side is at 54 and the high side went to 150 and then dropped to 80psi?

Answer: Since I don't know what system you have, I have to guess. Sounds like either a bad compressor or a bad H-block.

Question: 2020 mustang air works when it wants to. High side reading is normal but low side immediately pegged the gauge and didn’t move. Didn’t read anything on here about normal high side but high low side. What’s likely the culprit?

Answer: 2020 Mustang should be R1234yf refrigerant. Low side pressure is controlled by the H-block.

Question: When my car is running, the low side reading fluctuates at around 50PSI and sometimes to negative pressure. High side reading stays stagnant at 100PSI. Not-so-cool air is being blown out. Ambient temperature 30 degC. What could be the problem?

Answer: Not enough information. But the high side is extremely low. If the system is properly charged it seems the compressor is not working up to par.

Question: I have a 1998 suburban with rear a/c. The pressures are 30psi on the low side and 165psi high side. It cools to about 45 in the front and 60 in the rear. The compressor clutch stays engaged and the pressure readings stay the same. It has a new compressor, orifice tube, and accumulator. It had this problem prior to replacing these parts. I pulled a vacuum on the system prior to charging. Any thoughts?

Answer: Usually, the problem is reversed. The rear ac uses a TXV/H-Block and it sounds like the valve is not working. The systems with rear ac hold more refrigerant, usually about 3.8 pounds. Should be a tag on the radiator support.

Question: I charged the A/C system. The air inside cab is down to 50 degrees. Low side of Gauge reading is 40-50 psi. High side of gauge reading zero, but the cab is cool. I drove around for about an hour with the outside temperature 88 degrees. What is up with the high side?

Answer: The system can't be zero on the high side. There must be a connection problem with the high side coupler. The system should cool down to 40 degrees.

Question: I can't turn off my a/c. When it's switched on it works great. When I switch it off it still blows semi cool air. The switch is functioning correctly. Could the compressor clutch be partially sticking? The car is a 2006 Hyundai Accent.

Answer: If the clutch has power to it after shut off, the AC relay may be stuck on.

Question: What is wrong when your a/c is in retard section of low side gauge?

Answer: This means your low side pressure is above 150psi. System must be overcharged or your gauges are somehow connected wrong.

Question: My car reads 30psi low side at idle and high side reads 70-75 psi...no cooling effect...static pressure is 50psi... Ambient temperature is 30.c... Is my AC is undercharged?

Answer: Sounds undercharged. Leak?

Question: What causes the hig side pressure to be low? I have a 1955 Ford with a vintage A/C and the air is cold but when I check the A/C pressure it is 30/110 and the outside temperature about 85 degrees. Is that normal?

Answer: I assume it is an R12 system that has been converted to R134a. The lower high side pressure may be from an undercharged system and the TXV is keeping the low side at 30 PSI. The old systems used a sight glass to charge the systems. Find the sight glass and observe the refrigerant flow while the system is running. If the sight glass is clear liquid the system is fully charged. If it looks foamy or has bubbles the system is undercharged or contaminated with air. Since it does cool I would think it is close to being charged.

Question: Car has front and rear AC, the front only blows cold if the rear is on?

Answer: Does the compressor kick on with the front ac on only? If not front ac control problem.

Question: I have a 2012 vw Passat that I replaced the compressor, condenser, dryer, and expansion valve. Charged up to correct refrigerant specs of 575g. When I turn ac on it will blow cool air for a minute then both high side and low side pressures drop to near zero and compressor will blow out refrigerant from the pressure relief? Any suggestions?

Answer: Never seen this before. Sounds like a blockage in the high side line to compressor discharge oort.

Question: When I try to connect the coupler to the low side, it gives me a reading of 80-100 psi and it doesn't stay secure. What makes the coupler not to stay secure?

Answer: Wrong or damaged coupler would be my guess.

Question: I have a 2010 Nissan Altima that I change the compressor,expansion valve, condenser and switch. I vacuumed the system and put new refrigerant. It works but, the next day doesn’t work. I did this 2 times and pressure is the on the low and high?

Answer: Sounds like you have a leak. Without knowing static pressure this is a guess.

Question: I have a 1996 Dodge Ram Diesel. Low pressure was normal, but the high pressure was extremely high. Why would my Dodge's AC pressures be so different?

Answer: Sounds like poor airflow across the condenser. Cooling fan problem, plugged or blocked condenser are the probable causes.

Question: The A/C compressor has seized on my 2000 Sienna. I have to bypass with the belt. My mechanic says any bypass belt still will use the pulley or pass through the area of the compressor pulley. Does that make sense? Why would they call it a bypass belt if that is the case?

Answer: If the compressor seized, the pulley should still turn. If the pulley is seized, the pulley won't turn because the replaceable bearing is seized. Usually, to use a belt without ac, you need to remove the compressor as if it didn't have ac.

Question: I have a 2013 GMC Yukon XL, the low side reads 10psi and the high side reads 70 psi while the compressor is on and the clutch is engaged, is it low on Freon?

Answer: Sounds like it. Charge with DYE and retest with a black light.

Question: Why does the ac cut off every time I make a turn?

Answer: The computer senses the extra load added by the power steering and turns off the ac until the load is reduced. Typically used on small engines.

Question: I have a 1998 Monaco motorhome. I evacuated the system to get ready to add 134a. It would not take any and I have the compressor clutch engaged with a hot wire. Both gauges are reading zero. I'm thinking a bad compressor could this be right or something else?

Answer: Sounds like the gauges are not connecting. This is the reason you can't add refrigerant.


David Dai on September 01, 2020:

Hello Sir,

2013 Honda civic Hybrid

blowing not cold air, but after relay bypassed then work normally again. I have checked the relay and the control board, then replaced them with new ones, but the problems still there. could you have any suggestions for help me? Thank you.

Brian on September 01, 2020:

Hi Mike,

2017 Ford Transit 350 15 passenger mid roof van. Front and rear systems. I have lack

Luster cooling especially when it’s 110 outside here in Gilbert, AZ. Vent temps in the front are around 65 and rear is about 71. Yesterday when ambient temp was around 100 I had a low side pressure of 52 and a high side of 250. Based on some R134A charts I saw my high side should be over 300. Should I add refrigerant? Low side according to the chart should be 50-55.

Bill on August 23, 2020:

Hi, 2013 Fiesta ecoboost.

Low pressure is high 250psi, but high pressure is low 100 psi.

Compressor seems to work, turns & engine revs drop when ac on, but no cooling.

any ideas?

Victor from Abuja, Nigeria on August 18, 2020:

I m working on a 2011 model of a Kia cerato.. the AC cuts off when the vehicle accelerates even while the clutch still working. What could be the problem?

Mike Thomas (author) from Phoenix, AZ on August 07, 2020:

As soon as you start charging the low side, the high side pressure should go up and the compressor should kick on (with the engine running and AC on).

You need to figure out why those 2 things are not happening, Once the compressor kicks on, the low side pressure goes down and will draw in the refrigerant from the can. High flows to low pressure.

My guess is the old style TXV which I believe is under the dash is stuck closed or blocked by debris.

The compressor is sent power to operate. Why isn't it getting power?

Michael M from Concord on August 07, 2020:

Hi Mike. I started over as you suggested but evacuated for more like 45 minutes. I took better action notes this time... R134a can (with yellow hose + valve) weighed 17.3 oz. when I started recharge. It SLOWLY (over 30+ minutes) dropped down to about 9 oz. indicating about 8 oz. of refrigerant has gone into the system. Compressor had not kicked on yet. Gauges before starting recharge: Low -29; high 0. Opened low side only for adding R134a after purging yellow line. Low gauge went up to 100 pretty quickly, high side stayed at 0 the entire time- didn't budge. The really strange part is that after the can dropped by about 8 oz., it's apparently climbed back up (I walked away for a while) to 18 oz.. MORE than what it started at, and low pressure crept from 100 to 120+ over the 45 minute process. Clutch never kicked on as it did on my first attempt. Nuts!. Is it possible the system pushed coolant BACK into the can- more than what it started with? I'm so confused. Wondering if it has something to do with the converted system (from R12 to R134a long ago). When I removed the compressor there was almost no (mineral) oil in it- perhaps because it was dispersed throughout the system, but noted. I added the same amount of Pag 46 oil (7 oz) as the system called for in mineral oil- could the different oils translate to different amounts?- like they amounts are different for R12 vs R134a? It seems odd that 7 oz. is the same amount of PAG 100 called for in my big 98 Dodge 1500 v8 with beefier and longer AC lines, etc. Would too much oil cause problems like this? I also added 1/4 oz UV dye, FYI. I sure hope you have some idea what's going on here. Your expertise and help is much appreciated.

Mike Thomas (author) from Phoenix, AZ on August 03, 2020:

I would recover, evacuate and recharge before doing anything else. Evacuate for 30 minutes. Then recharge to the proper amount of refrigerant. Doesn't make sense that the low side is higher than the high side.

Michael M from Concord on August 03, 2020:

Hi Mike- thanks for this great info. Can you help me?

93 Toyota Pickup 2WD M/T. I thought the compressor locked up but apparently it was just the bearing on the pulley since the clutch turned with resistance- only the pulley was seized up. BUT since I had the new compressor... I replaced the clutch and accumulator after flushing the entire system. Good vacuum for 24 hours. When I to to recharge with R134A, the system isn't drawing in freon from the can... maybe 8 oz. only, enough to engage the clutch/compressor.

At static/off/cold, the low/high pressures stabilize at about 38 each. However, with engine/AC on, the LOW side pressure worked it's way up to around 115 with high side at 75.

Your article has notes about "Higher Low Side and Lower High Side Pressures" indicating too much refrigerant is flowing into the evaporator. I did not replace the expansion valve since the AC was working fine before the pulley bearing locked up and there weren't any clogs when I flushed the system out (solvent/air flush kit).

Guessing you'll advise to replace the expansion valve too, but how do I "start over" in case I goofed somewhere- to double check everything before I commit to replacing the expansion valve?

Thanks again!

Mike Thomas (author) from Phoenix, AZ on August 01, 2020:

The high side cannot be 0.

Is there power(12v) to the clutch?

Yes, check ground and clutch resistance (1.5 to 3 ohms).

No 12v means no power.

Need to check for voltage and ground with a meter.

cyndi on August 01, 2020:

cant figure out whats wrong with my 2007 honda oddessey a/c. i have replaced the a/c compressor/ clutch, high low pressure switch, new a/c relay switch , added freon, checked gauges and low side 100psi but high side zero. clutch is not turning on. noone knows what else to do .. please help

Jarod on July 28, 2020:

Hello, I Just replaced the front seal in the compressor because of a leak. I replaced the accumulator and the orifice tube while I was in there. At Idle on a 92 degree day it has high low side pressure at 65-70 psi and a high side pressure of 250-275 psi. I am getting cool air at idle but far from cold. Obviously without air flow and RPM it is going to be a bit warmer. The week before it was the same ambient temperatures and it was much colder at idle. I did vacuum the system and it held 29 in hg for 24 hours. I did add about 2 ounces of oil for what I may have lost. But I am really wondering if it has a oil overcharge could this be it. I recovered it and checked the orifice tube but it looked fine except for oil and Dye on it which I cleaned off. It does pull down to about 55 psi when holding the throttle at 2000 rpm. Also there are no airflow blockages.

Mike on July 24, 2020:

Check the ac condenser and fan operation. Sounds like an airflow problem.

Jeevan on July 24, 2020:

I have 2016 civic. My ac works good but after 10-15 mins of driving (mostly highway speed) ac starts blowing less cold air. Then I had to turn off ac and turn back on. Then it works again. This cycle continues.

Few weeks ago I used dye to do leak test and before that my ac was good.

This is R1234yf

Mike Thomas (author) from Phoenix, AZ on July 13, 2020:

Pressures are high? What is high? Sounds like a restriction in the high side. See if you can feel where the line goes from hot to cold or just warm. This happens often when a compressor goes bad.

Jennifer on July 13, 2020:

I have a 2014 Altima. My readings are high, but only after I add about half a can of freon. I've changed the condensor, expansion valve and vacuum. I've also vacuumed the system and the air is still hot and the pressure still getting high. Im stuck, please help.

Michael M. Thomas on July 12, 2020:


If you are not allowing air in, I would check the H-Block and High pressure switch that signals the fans on.

KennethCW on July 12, 2020:

I have a 2007 Nissan Altima.

Vacuum the system to -30. Charged by weight, first can 12oz, low side 25-30 psi with high side at 150 psi. Slowly started second 12oz, stopped at 5.6 oz because the low side would climb to 45 and drop to 25 very quickly, and repeat, while the high side dropped to 100 psi and stays there. During the drop the fans kick on. During the rise they cycle off. The compressor is on during all of this, never cycles on or off… at least I do not hear it nor does the center hub stop spinning. Two different compressors have been tried. Same results.

Mike Thomas (author) from Phoenix, AZ on July 08, 2020:


With the system off pressure should equalize about 100 psi at your temperature.

When started low side cycles between 45 and 24psi.

High side should go up above 175psi.

Mat on July 08, 2020:

Put my gauges on 2015 Chevy pickup and have 30 low side and 200 high side 90F ambient. No cool air. Low side line isn’t even cold or condensing but compressor is clicking on and off when I cycle it. Also the gauges go down when you start up the vehicle so one would think the compressor is pumping. Any ideas?

Mike Thomas (author) from Phoenix, AZ on June 27, 2020:

On the GMC I believe you pull the HVAC /ECAs fuse under the hood for 1 minute. Turn on the key and let it reset for 1 minute. Turn off the key the start it up. Should have reset.

JBlythe on June 27, 2020:

I have a 2012 GMC Sierra. This evening I accidentally left the key in "on" position and door open with dome light on & headlights on automatic. (engine not started and running) After being in the house 1-2 hours I noticed dim light out side. Went and checked, dead battery. Jumped and started and let run for 30 minutes taking a charge. Went to get something to eat and had hot air blowing out 2 of the 4 in dash vents. Cold air blowing out of the other 2 vents. (Hot air blowing out of the furthest left/drivers side and hot air blowing out of the left vent of the middle/center vents & nice cold air blowing out of the right center vent & far right vent (passenger side) Confusing, I would not think the center 2 vents would have duc-work for each vent, 1 blowing hot & other cold. Crazy! I've never seen anything like this. The AC in this truck has always been little Siberia. I don't see where dead battery and jumping would be connected to a vent issue. By the way, compressor kicking in (cold).

Any Ideas?

Appreciate it,

J. Blythe

Mike Thomas (author) from Phoenix, AZ on June 08, 2020:

99 GMC has CCOT compressor cycling orifice tube system. Compresser cycles, turns on at 45psi and off at 24psi.

curtisbrown1 on June 07, 2020:

99 GMC Suburban

ok with the engine off the gauges read 100 high and Low

Engine running both front and rear Low-45 High 250-275

Engine running Front only turns off at Low 25 High 200 turns back on Low 45 High 150

Daniel Cantwell on March 28, 2020:

Ok. Thanks for the info I will follow up in a day or two.

Mike Thomas (author) from Phoenix, AZ on March 28, 2020:

With the engine off (static) the pressures should equalize. Sounds like the pressures are low indicating low refrigerant charge. This means a leak. Recover, Evacuate and Charge the system. Add black light (ultraviolet) dye before charging. Charge the system and run for a while. Check for leaks with black light. It may take a while (Days) for the dye to show up depending on leak size.

Daniel Cantwell on March 28, 2020:

Static pressure on low side is 50 high side is 20

temperature along southern coast of NC is 62*

Mike Thomas (author) from Phoenix, AZ on March 27, 2020:

Not enough information.

Static Pressure and Temperature ______________________________

Low and high side pressure. _________________________

Daniel Cantwell on March 27, 2020:

Great page great info

05 TSX ac ice cold last summer into late fall. This spring warm. Low pressure side high90-100 needle far left high side 65 needle far right.

Andy Jones on October 21, 2019:


I took my 2006 Mazda 5 in for a diagnostics as it hasn't been blowing an cold air for past few months. They did a diagnostics and say the high side pressure is too high and suggest taking it back for a flush, clean and re-gas. The funny thing is since the diagnostic it working fine. Do you think I need to get the work done or can I get away without it? The car is old so reluctant to spend too much on it if I can get away with it.



Mike Thomas (author) from Phoenix, AZ on October 09, 2019:


Sounds like the blend door is not sealing all the way. Something may have fallen down inside the plenum.

Terri Kim on October 09, 2019:

have a 2013 GMC Acadia with dual temp system.

passenger side is not blowing cold air.

checked the blend door actuator and it is working fine.

removed the actuator and manually turned the door. air goes from hot to cool air but never as cold as the driver side.

what are other possible causes beside the blend door?

thank you

Mike Thomas (author) from Phoenix, AZ on September 29, 2019:

If the system is prooerly charged, sounds like a bad compressor

robb on September 29, 2019:

need help with s-10. pressure low side 84lb. pressure high side 150Lb. blowing warm

Dale from South central Oregon on September 02, 2019:


I have an '06 Dodge cummins. A/C was working fine, turned off engine for 10 mins. and started engine again but A/C refused to cool.

I layed underneath and could turn clutch by hand, ¼ turn or so of clutch felt like I hit a detent. It's not a smooth feel as I rotate clutch by hand.

I'm sure that's not normal.

richard scott on August 02, 2019:

really helpful chart. decided to pursue my ase. all the way to master and b. degree. gonna start with ac. sounds like you know what your talking about

Mike Thomas (author) from Phoenix, AZ on July 03, 2019:


I would start from scratch and drain evacuate and recharge the system with the correct amount of refrigerant. This will make sure there is the right amount of refrigerant and no contaminants (air).

I assume this is a Pickup so no rear ac. If this is a TXV the compressor doesn't cycle.

Doug on July 01, 2019:


Have 03 Dodge Ram

My static pressure was good

80 at 77 degrees

But operating temps were low

25 low

And 125 high.

Hooked up refrigerant and before I added

The clutch wouldn’t disengage

And high side went to almost 400.

I let it sit

Let some refrigerant out and in the end got to 45 low


Clutch would engage and only low side would move.

Turned off and static pressure is 78 low

And 125high after sitting.

The high side will not move ever

Mike Thomas (author) from Phoenix, AZ on June 19, 2019:


The ac light blinking means the computer has found a fault. This is usually an electrical problem, like a bad sensor. The high side pressure should be equal to or higher off and running.

Mike Thomas (author) from Phoenix, AZ on June 19, 2019:

Sounds like a vacuum actuator problem. May have an electric vacuum pump or vacuum leak problem.

Mike Thomas (author) from Phoenix, AZ on June 17, 2019:

Bean Motorhome

Sounds like the system is undercharged and the compressor is either not kicking on or a bad compressor.

Luke on June 16, 2019:

Hello I have a 2010 tundra with a/c light blinking and compressor not turning on.

Already has a new compressor and a/c relay.A/C gauge reading was low side 90psi and high side 50psi.just wondering if you have any idea.

Dale McLean on June 12, 2019:

Have a Ford Triton V-10 engine in my motor home. The air conditioning works fine on trips but after about 30 minutes the air stops coming from the dash vents and is diverted to the defroster vents for about 1 minute (varies), then back to the dash vents for maybe 4 0r 5 minutes . After it starts it continues this cycle for the duration of the trip.

Bean Motorhome on June 08, 2019:

Have an 2008 Tiffin Allegro Bay RV basically the same AC as any car (I'm Guessing)

Bought a set of gauges (but first did the cheep way fro Walmart a recharge kit) but found my high side is 50(should be around 250) and low side is 45 and per spec's not sure whereto start to look any advice planing a trip from FL to CT and need ac to work and would appreciate and suggestions where to look

Mike Thomas (author) from Phoenix, AZ on May 12, 2019:

Any time you are losing refrigerant, you should add black light dye on the next recharge. After running the ac for awhile, use the blacklight to find the leaks. the leak will show up as yellow green because it will be carried out by the oil leaking out. On small leaks it may take a week or so of running, sometimes longer.

Mike Thomas (author) from Phoenix, AZ on September 27, 2018:

How hard are you accelerating?

Max accel ca. Shut off the ac but should return when you let off the gas.

May be a loose connection. Try moving the wires with it running.

Ali on September 27, 2018:

Hello, I own a bmw X1 E84 and I have an issue with AC. When I accelerate, the AC doesn't blow cool air and I noticed that the cooling fan stops spinning. I have to restart it to get it working again.Please would you tell me what could be the problem. Thanks in advance. Have a good day.

Jeremy on September 17, 2018:


Thanks for taking the time to share your knowledge, It was a bad relay on the Honda. I was also having the same problem with my 93 Toyota pickup, started looking around and found a hidden in-line fuse that went bad, fixed them both.

Thanks again,


Mike Thomas (author) from Phoenix, AZ on September 16, 2018:

If the center clutch hub is turning the pressures should go up on high side and down on low side. Sounds like bad compressor.. when the compressor kicks in ios there any change engine RPM?

Mike Bittner on September 16, 2018:

2003 honda element. New compressor,new condensor ,new expansion valve. Compressor kicks on but the pressures remain the same .They do not change.Low and high side remain the same.got me stumped.Could it be a bad new compressor.I did vacuum for 30 minutes.


Mike Bittner

Mike Thomas (author) from Phoenix, AZ on September 15, 2018:

If the pressure is at 250psi without the compressor running the underhood temperature could have raised the static pressure if it was hot from running the engine. At 100 degrees, the static pressure should be 125psi approx. if the vehicle has cooled off.

The compressor doesn't engage, sounds like the problem. The clutch doesn't click in? Check the electrical for the compressor.

Jeremy on September 14, 2018:


Wife was driving the car,a/c stopped blowing cold, threw the gauges on it, low side pegged out retard, high side around 250, around 100* outside. Compressor does not engage, spins freely by hand, static pressure is also pegged out on low side and 250 on high. Do you think the expansion valve is stuck open or check valves in the compressor. Thanks

2010 Honda odyssey

Mike Thomas (author) from Phoenix, AZ on September 01, 2018:

Too much oil takes up space that should be refrigerant.

Oil is thruout the system.

You may try lowering the amount of refrigerant by 15%, GM used this in retrofits when old oil was left in and new oil was added.

The other option is pulling the compresser drain and measure the oil. Add recommended amount. That still doesn't tell you how much is in the rest of the system.

NC Mike on September 01, 2018:


I have a 2004 Honda CRV. Compressor was replaced, but old was not exploded. Drier material and expansion valve were replaced. Upon initial startup, the compressor appeared to be locked up with belt slipping on it. After shutdown and re-start of vehicle, the compressor cycled as normal once the proper refrigerant amount was reached. The cabin temperature was never great, just below where it should be and barely keeping up. The condenser fan was not kicking in, so it was replaced.

So now for the problem. The compressor clutch now does not appear to be cycling on/off, and the temperature is still not very cold at vent. Low side line is cold to touch where it should be, and high is very warm. I then checked the resting pressure of system, and the low side pegs out all the way to the highest point. The high is around 200. I do think there is more oil than was called for when installing the new pump: Like an idiot, I forgot to empty and replace a metered amount into the pump. Could it be too much oil? Is this a clutch issue or compressor issue? Can you help?

Mike Thomas (author) from Phoenix, AZ on September 01, 2018:

Not enough information but, sounds like a stuck open TXV/H-block.

agb3 on August 31, 2018:

low side is 85 high side is 150. any suggestions?

Mike Thomas (author) from Phoenix, AZ on August 17, 2018:

Chevy trucks use electric actuators. There is a left and right temp control. Try cycling the passenger temp lever from hot to cold several times. It should switch to heat then cold. If not the actuator probably needs to be replaced

Harvey on August 17, 2018:

2004 Chevy truck is blowing hot air from passengers vents

Mike on August 15, 2018:

I don't know about your high side but it sounds like a fan problem.

jglassaz on August 15, 2018:

2007 buick rendezvous

everything in the system has been replaced except the a/c lines and pressure sensors. compressor, evaporator, condenser, TXV and drier. I've evacuated the system. Put freon in. The system runs but not very cold. After running for 10-15 min the compressor stops and the low side has pegged the needle and there aren't any readings from the high side.

Mike Thomas (author) from Phoenix, AZ on August 08, 2018:


At 90 degrees the low side should be 29 to 31 and the high side 200 to 275 typically. There is no adjustment. The Hblock TXV controls the flow into the evap at about 30 psi.

Teo on August 07, 2018:

My subcool is lower than superheat, is undercharged. My ambient air temp (Outside air temp) is 90F degrees, the superheat should be 8F-12F degrees. My problem here is I need to refill the refrigerant so my low side is around 35psi. But no matter how I refill, it won’t increase, still stay at 30psi. For your info, my high side is 200psi. My gauge(checked) is ok. Can I know whats the problem. Thanks.

Mike Thomas (author) from Phoenix, AZ on August 07, 2018:


No pressure change means either the compressor is not turning on or bad compressor.

Mike Thomas (author) from Phoenix, AZ on August 07, 2018:


search goggle and Youtube

You can also buy a Mitchell or if you know someone with access look up the procedure.

Devin on August 07, 2018:

Hissing noise then you star the car. Refrigerant inidcates full and ac lines are about the same temperature.

Mike Thomas (author) from Phoenix, AZ on August 07, 2018:


The pressure are wrong.

It could not be higher on the low side.

Mike Thomas (author) from Phoenix, AZ on August 07, 2018:


If the low and high side are about the same, the compressor is not working.

The high side should go up and the low side down when the compressor kicks on.

Michael Day on August 07, 2018:

I have a 99 Honda Civic the low side is 350 and the high side 150.And still blows cold air out 44 degrees

jglassaz on August 07, 2018:

I'm trying to remove the dash to change out the evap core. Haven't been able to find any diagrams.

Vehicle is 2007 Buick Rendezvous CXL 3.5l V6


Scott Johnson on August 05, 2018:

I put my air conditioning unit on my gauges And the low side buried the needle all the way to the right And my high side is about 150 and the needle will climb to to 170 and stay there.

Mike Thomas on July 30, 2018:

Without knowing the vehicle or system type, it sounds like a restricted or sticking H-block.

Jrod on July 30, 2018:

It is random with the engine at around 1500 rpm. Sounds like it's coming from the cowl around the expansion valve area

Mike Thomas (author) from Phoenix, AZ on July 30, 2018:

Not sure without hearing it.

Does it change with engine rpm or if you turn ac button on off.

Could be compressor noisey.

Jrod on July 29, 2018:

My ac is making a fog horn noise after it had run for a while. Any ideas?

Mike Thomas (author) from Phoenix, AZ on July 24, 2018:


The temp of the lines depends on the pressure inside. 75 degree day the line will be real warm to touch, 95 degree day it will be hot to the touch because high side pressure is higher.

The low side should feel cold if everything is working no matter the outside temp.

Steve Crow on July 24, 2018:

How hot should the high side line be if you put a IR temp device on it? 140 to 200°

Mike Thomas (author) from Phoenix, AZ on July 23, 2018:


If your gauge readings are accurate, there seems to be blockage between the low and high side. This could be a bad TXV/H-block or blocked Orifice Tube depending on your system.

Teo on July 23, 2018:

My car AC static pressure is not equal. The low pressure side is 80psi and the high pressure side is 105psi. The ambient temperature is 32 degree celsius. Actually I should get both side 105psi. Can I know what is the problem?

Mike Thomas (author) from Phoenix, AZ on July 21, 2018:


The readings seem to be very high for the system not running. The ambient temperature affects the readings and a hot engine compartment could also raise temperatures. Overcharging the system will also raise pressures. More information is needed.

Ambient temp

Engine Compartment temp

Refrigerant charge

David on July 20, 2018:

2011 Toyota Avalon

Just took readings as compresso clutch was not engaging. High side reading at about 130 and the low side was between 120 and 350 in the retard zone. Expansion valve?

Mike Thomas (author) from Phoenix, AZ on July 13, 2018:

Greg your VW Rutan also sounds like a fan or blocked condenser problem. If driving at higher speed seems to allow the sysyem to work, that would confirm it.

Mike Thomas (author) from Phoenix, AZ on July 13, 2018:

Sounds like a fan problem or blocked condenser.

Gabe on July 13, 2018:

I recharged my a/c, wirked for about 10mins and went hot. Now the pressure on the line is past the red, its trying to make a seond trip around the guage. Any clues!

Mike Thomas (author) from Phoenix, AZ on June 12, 2018:


90psi should be enough static pressure (depending on outside temp) to operate the system. Your other pressure readings are not correct. It seem like you have 2 low side pressures. Rehook the gauges, system off, and make sure the valves are closed and the knobs are both turned clockwise on the connectors to open the fittings. The high side should go up and the low side down from the static readings with the compressor running.

Since the system works some times, there is an intermittent problem causing the problem or low air flow across the condenser

Mike Thomas (author) from Phoenix, AZ on June 12, 2018:


Sounds like a fan problem or blocked condenser fins. Does the AC work better on the highway verses intown stop and go?

Richh52 on June 12, 2018:

My a/c works part time. I checked the refrigerant pressures. When the compressor runs high side and low side are the same at 42lbs. When the compressor is shut off the pressures are the same at 90lbs. Could this be due to a faulty valve or is it the compressor. When it is working, it is very cold.

David D on June 12, 2018:

R134a at 95 degree F should be 50-55 low side and 275-300 high side. When I check static pressure, it takes 15 minutes for gauges to equalize at 130 psi. With compressor on the initial readings are both low 30/250 and creeps to 27/235 blowing 42 deg F air. After about 20 minutes low reads low and high reads high 32/320, blowing 52 degree air, Increased load from AC causes Tentionier/Ribbed belt to vibrate violently. It this a partial blockage or is it freezing up somewhere creating a blockages or poor cooling fans? What is your guess?

Mike Thomas (author) from Phoenix, AZ on May 30, 2018:

Jack Leg

Glad you found your problems. Ask the shop if they put dye in with the refrigerant. If they did, use a black light flashlight and the leak will show up bright yellow/green. Depending on the size of the leak it may take several weeks to show up.

Jack Leg on May 29, 2018:


As promised - follow-up. Two problems. First, fan module was kicking in and out causing the fan to work only sometimes. In 92 degree heat fan finally quit completely and got a check engine code for overheat. Direct wire to fan showed fan worked OK. So, replaced fan module with junk yard module ($25 for part). Problem one solved. Second, as you suggested, at a good shop had system completely recovered, evacuated and system held vacuum ok. Took 11 oz of 134a (27 oz total system). So, it was pretty low. Cost - $120 including the 134a (cost was much better than buying a recovery unit). Might have a very small leak, but if it lasts 6+ months it's better than the cost and effort to fix the leak. Thanks again for the pointer.

Mike Thomas (author) from Phoenix, AZ on May 27, 2018:

Warm mailman

It sounds like your vehicle has dual zones. There is a separate actuator for each side. If you have dual controls try moving it from

Cold to Hot (blue to Red). Should feel warm air on hot and cold on Cold. Try several times. If that doesn't work the actuator need to be replaced.

Mike Thomas (author) from Phoenix, AZ on May 27, 2018:

Red hill

Sound like the compressor is being shut off by the computer or a bad connection. If the system runs for a short time and shuts off, I would check for cooling fan operation.

warm mailman on May 27, 2018:

what would cause driver a/c vents to blow cold but passenger vents to blow warm

red hill on May 26, 2018:

my system starts out cold then the a/c clutch disengages .relay or clutch ?

Mike Thomas (author) from Phoenix, AZ on May 14, 2018:

Is the accumulator cold with the system running? If it is it could be the blend door actuator in the heat position. If it is barely cool or warm it is an ac system (low Refrigerant) problem.

Jack Leg on May 14, 2018:

Thanks Mike. I'll have to get that done outside. I don't have the recovery unit. Happened all of a sudden so there might be a leak developed that is big enough to cause a sudden issue. Don't see any evidence of a leak, but visual is no guarantee.

I will let let you know the result. Jack Leg.

Mike Thomas (author) from Phoenix, AZ on May 12, 2018:

With the temp being 85 to 90 degrees the static pressure should be higher. This indicates low refrigerant. The quick cycling of the compressor is also an indicator. Recover the system and measure the refrigerant. I would bet it is less than half charged.

Jack Leg on May 12, 2018:

Mike, great article on gauge reading.

Question: 2008 Merc Grand Marq, a/c system is the completely auto type. R134a system - Here's the readings: Engine off , a/c off - System static pressure is High side 75, Low side 85. Engine on and a/c running on max cooling - High side 175, Low side 25 to 28 (the compressor is drawing down ok on low side when it cycles). Ambient temp is 88 to 90 degrees, about 65% to 75% humidity. R134a does not appear to be low. Here's the problem: Engine running and a/c on max - blower works fine, but the compressor kicks on for about 5 seconds, then kicks off for about 15 to 20 seconds. This cycle just keeps repeating. Clutch looks OK, not heating up so compressor appears to engage properly when it cycles on. However, no cooling at all - the compressor does not cycle on long enough - dash vent temp is about 100 to 110 degrees. I think I'm OK with the refrig part of the system, but I'm not experienced with the automatic electrical stuff in the newer type cars. My guess is some type of sensor or relay causing the compressor to cycle on and off rather than anything with actuators...your diagnosis and suggestions of where to start looking would be much appreciated. Thanks in advance. Jack Leg DIYer

Mike Thomas (author) from Phoenix, AZ on May 10, 2018:

Glad I can help.

Salina Hocutt on May 10, 2018:

Thank you for sharing this information. I am a beginner and don't have much knowledge about this stuff.

Mike Thomas (author) from Phoenix, AZ on June 01, 2017:


The pressures for all 3 refrigerants are very close.

With TXV or H-Block about 30psi on the Low Side and 125 to 275psi on the High Side depending on temperature outside.

MAC on May 31, 2017:

The Readings depend on the type of refrigerant, did you mention the type, I don't see it?

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