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My Car Temperature Gauge Reads High

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

The reasons behind a car temperature gauge reading high may include:

  • Low coolant in the system
  • Incorrect coolant for the application
  • Cooling system problem
  • Bad temperature gauge

Some vehicle models use a warning light instead of a gauge. Whether the gauge reads high or the light comes on, it means engine driving conditions are not safe, depending on coolant temperature.

On most vehicle models, the normal engine operating temperature falls between 200F and 220F (90C and 105C).

Keep in mind, though, there are driving conditions that will cause a temperature gauge to read above normal. For example:

  • Driving uphill
  • Towing a trailer
  • Driving in slow moving traffic
  • Idling at high ambient temperatures and AC on
  • Driving in a high altitude geographic area

If you are not driving under any of the previous conditions, check the following sections describing the most common problems causing a gauge temperature to read high.

To help in your diagnosis, have the repair manual for your particular vehicle model on hand. If you don't have this manual yet, buy an inexpensive copy through Amazon.

Haynes manuals include:

  • Photographs
  • Step-by-step repair procedures
  • Components location
  • Diagnostics
  • Electrical diagrams
  • Maintenance schedule
  • Components' specs

The manual will be of great help with your car maintenance projects at home, so you can save a lot of money.

In This Article

1. Do You Have Enough Coolant In the Overflow Tank and Radiator
2. Is the Radiator or AC Condenser Block Dirty
3. Is the Radiator Low in Coolant But Not the Overflow Tank
4. Is the Electric Cooling Fan Turning On
5. Is Coolant Flowing
6. Are the Radiator Cap and Neck in Good Condition
7. Did You Mix the Antifreeze and Water Correctly
8. Is the Thermostat Working Properly
9. Do You Have a Faulty Temperature Sensor or Gauge
10. VIDEO: How to Test a Coolant Temperature Sensor
11. What to Do If Your Engine Overheats While Driving
12. Resource

Make sure you got enough coolant in the overflow tank.

Make sure you got enough coolant in the overflow tank.

1. Do You Have Enough Coolant In the Overflow Tank and Radiator?

Coolant system leaks will lead to overheating issues. If you've been adding coolant to the radiator or overflow tank, this is a sign of a potential leak.

Inspect the cooling system components for wet areas, even the floor where you park your car at home or work.

Besides the cooling system, check the area around the cylinder head gasket and heater core for possible leaks as well.

Consult your vehicle repair manual, and the Resources section at the bottom of this post for more help.

Remove buildup from the front of the radiator block.

Remove buildup from the front of the radiator block.

2. Is the Radiator or AC Condenser Block Dirty?

Remove dirt, dry leaves, bugs, and other debris from the front of the radiator and AC condenser.

Over time, the accumulation of debris reduces airflow, hindering the radiator fins' capacity to remove heat.

You can use a brush with stiff bristles to clean the radiator core. Also, if some of your fins are bent, you can use a special radiator comb tool, that will help you fix and clean the fins at the same time.

Check coolant level in the overflow tank.

Check coolant level in the overflow tank.

3. Is the Radiator Low in Coolant But Not the Overflow Tank?

First, check the radiator cap and radiator neck (see section 6).

Then, closely inspect the hose between the radiator neck and the overflow tank.

  • Make sure the hose is tight at both ends.
  • Check the hose for kinks or tears.
  • Check the hose for blockages.
  • Inspect the overflow tank for blockages.
Make sure the cooling fan is working.

Make sure the cooling fan is working.

4. Is the Electric Cooling Fan Turning On?

A cooling fan may fail to come on, or may work erratically.

Make sure the cooling fan runs when the engine reaches operating temperature.

Otherwise, your vehicle repair manual can help you diagnose the problem. Also, check the Resources section at the bottom of this post for more help.

Warning!

Never remove a radiator cap when the engine is hot. Very hot coolant may squirt out of the radiator and seriously burn your skin.

Check radiator for internal restrictions.

Check radiator for internal restrictions.

5. Is Coolant Flowing?

Verify that coolant is flowing through the system.

  1. Remove the radiator cap when the engine is cool.
  2. Start the engine.
  3. Wait for the engine to reach operating temperature. This may take about 15 to 20 minutes.
  4. Through the radiator neck, verify that coolant is flowing.
  5. If coolant is not flowing:
    • Check for a bad thermostat.
    • Check for a bad water pump.
    • Check for radiator restrictions.

Consult your vehicle repair manual, and the Resources section at the end of this post.

Check the radiator cap seals and neck sealing surfaces.

Check the radiator cap seals and neck sealing surfaces.

6. Are the Radiator Cap and Neck in Good Condition?

Make sure the radiator cap is installed tightly, and check the cap seals as well. Then, check the radiator neck sealing surfaces for buildup and damage.

You may want to test the cap pressure using a cooling system pressure tool. You can borrow this tool from your local auto parts store.

Look up the corresponding pressure rating for your system in your repair manual.

If necessary, replace the cap or repair the radiator neck.

Mix the correct amount of antifreeze and coolant.

Mix the correct amount of antifreeze and coolant.

7. Did You Mix the Antifreeze and Water Correctly?

If you recently replaced the coolant in your car, make sure the mixture is correct.

Normally, a 50/50 mix is used for the most weather conditions. For example, in a cooling system with capacity for four gallons, you'll mix two gallons of antifreeze with two gallons of distilled water.

You can use test strips (litmus paper) to measure the antifreeze/water mixture. You can find litmus paper in most auto part stores.

However, if you haven't changed the coolant in more than two years, it's better to flush the radiator and replace the coolant to remove buildup and make sure the system is properly protected against corrosion.

Consult your vehicle repair manual for the recommended antifreeze, coolant mixture, and capacity for your application.

A thermostat can fail after years of service.

A thermostat can fail after years of service.

8. Is the Thermostat Working Properly?

A thermostat may partially open or remain closed, preventing coolant flow, and causing the engine to overheat.

Check the thermostat operation using your vehicle repair manual. You'll also find help in the Resources section at the end of this post.

Check your coolant temperature sensor.

Check your coolant temperature sensor.

9. Do You Have a Faulty Temperature Sensor or Gauge?

What if the cooling system is working fine, or you've determined that the engine doesn't appear to be running above operating temperature?

You may want to check the engine coolant temperature (ECT) sensor and, if necessary, the temperature gauge.

Most modern vehicles rely on the ECT sensor to run the temperature gauge or the coolant overheat warning lamp. Older models use a separate sensor.

Usually, car technicians use a scan tool to read engine temperature through the ECT and compare this result to an actual engine temperature reading using an infrared pyrometer or contact-type temperature probe.

However, you can test ECT sensor operation, or the one used by your temperature gauge or light, using a digital multimeter.

Consult your vehicle repair manual for the proper procedure. Also, you'll find help in the Resources section at the end of this post.

10. VIDEO: How to Test a Coolant Temperature Sensor

The following video shows you how to locate the engine coolant temperature sensor in most cars, and how to test it. If you believe there's something worn with your ECT sensor, this video can help you test it.

Also, look up the test procedure in the repair manual for your particular vehicle model. You'll also find some help in the Resources section at the bottom of this post.

When your engine overheats at speed, pull over to prevent serious damage.

When your engine overheats at speed, pull over to prevent serious damage.

11. What to Do If Your Engine Overheats While Driving

When your temperature gauge reaches the red line, or the temperature warning light comes on, while driving, engine damage can occur. This usually means your engine has reached above 250F (120C).

The following procedure will help you deal with an overheating engine on the road.

  1. If your air conditioning is running, turn it off.
  2. Set your heater to the MAX setting and turn it on to help remove some of the heat in the engine. Roll down the windows, if necessary, and direct the center and side vents toward the windows.
  3. If you are at a stop light or stuck in heavy traffic, set the transmission to Park or Neutral and raise engine speed to increase coolant circulation through the radiator.
  4. Pull over and turn the engine off as soon as possible. Wait for the engine to cool.
  5. Pop the hood open and confirm the engine is hot or smells hot.

    If the engine is hot, you may see the coolant boiling in the reservoir tank. Wait for the engine to cool. You may need to wait for about an hour before you may attempt to move the vehicle. You may want to bring the car to a nearby car shop or call a tow truck to have the engine serviced.

    If the engine doesn't appear to be overheating, the problem might be with the gauge or coolant temperature sensor. Consult your vehicle repair manual, and the Resources section at the end of this post.

12. Resources

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.

© 2022 Dan Ferrell