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Diagnosing Trouble Codes P0171 and P0174

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

Carbon buildup around the throttle body bore and valve may cause fuel delivery problems.

Carbon buildup around the throttle body bore and valve may cause fuel delivery problems.

Car Trouble Codes

A trouble code P0171 or P0174 means the oxygen sensor or Air-Fuel Ratio (AFR) sensor (in bank 1 [P0171] or bank 2 [P0174]) has detected a lean fuel mixture: either too much air or not enough fuel is going into the engine. One or both of these lean-fuel condition codes can be set for several reasons:

  • Inaccurate information from the MAF sensor (dirty or bad sensor)
  • Air cleaner assembly leak
  • Clogging fuel filter
  • Worn fuel pump
  • Dirty or plugged fuel injectors
  • PCV system leak
  • Failed fuel pressure regulator
  • Vacuum leaks
  • EGR system issues
  • Oxygen sensor problems

Also, although less common, a problem in the powertrain control module (PCM), the car's computer, can cause these codes. Either the PCM has failed or its software needs an update.

When You Have Codes P0171 and P0174, You May Also Notice:

  • Rough idle
  • Lack of power
  • Misfiring
  • Hard starting

When they get codes P0171 and P0174, drivers often assume that the oxygen (O2) sensor has failed. It may have, but the oxygen sensor or AFR sensor (gasoline direct injection [GDI] and other engines) is not as common a cause of these codes as the other sources of trouble listed above.

So before you replace an O2 sensor in your car, check the following sections to avoid wasting money and time.

One more thing. Before you start any tests, make sure all the air ducts and vacuum hoses are properly connected. Loose or improperly attached ducts and hoses are a common source of lean codes.


1. What You Need to Know About Lean Fuel Codes

2. MAF Sensor and Throttle Body issues

3. Vacuum Leaks

4. PCV system issues

5. Fuel System issues

6. Oxygen sensor issues

7. Preventing Damage from Trouble Codes P0171 and P0174

Trouble code P0171 points to bank 1 (cylinder 1 side) of the engine.

Trouble code P0171 points to bank 1 (cylinder 1 side) of the engine.

1. What You Need to Know About Lean Fuel Codes

The Difference Between Trouble Codes P0171 and P0174

Basically, a P0171 code points to "bank 1," the side of the engine with cylinder number one, while P0174 points to the cylinders on the other side of the V6 or V8 engine.

If your engine has four or six cylinders inline (that is, all on one side of the engine), most likely you'll get a P0171 code. However, on some newer vehicle models, the computer treats the engine as two separate banks when generating codes, even if it's configured inline. For example, in a four-cylinder engine, bank 1 represents cylinders one and two, while bank 2 represents cylinders three and four.

If you need more information about your particular model, consult your vehicle repair manual.

What if I Have Other Codes?

Often, a P0171 and/or P0174 code will be accompanied by a P0300 random misfire trouble code. In this case, concentrate on the P0171 or P074 code, since they are the most likely cause the engine is experiencing a random misfire.

If your computer logged both P0171 and P0174 codes, concentrate on those components affecting all cylinders, like a MAF sensor, a clogged fuel filter, or issues with a fuel pressure regulator. Otherwise, concentrate on the side of the engine represented by your diagnostic trouble code.

Just be aware that problems with a MAF sensor sometimes trigger a single code even though they affect all cylinders.

Using a Scan Tool

A P0171 or P0174 trouble code can happen because of a vacuum leak or faulty sensor. Vacuum leak faults usually—not always—manifest more readily at lower speeds; as a rough idle, for example.

Trouble at higher engine speeds is more common. Trouble at higher speeds is often caused by a bad sensor or a problem in the fuel system. Sensors that can go bad include the MAF sensor, engine coolant temperature (ECT) sensor, throttle position sensor (TPS), barometric pressure sensor, or other sensors that control fuel delivery

A capable scan tool can check fuel trim values at idle and at higher engine speeds and detect when the lean condition occurs. This can be of great help in locating the source of the problem.

If your computer has other trouble codes pointing to one or more sensors, check those first. They may be the cause of the P0171 or P0174 codes.

If you get a code for an oxygen sensor, it may or may not be the cause of the fault; more often the other sensors are the problem. So you may want to check other components first before testing and replacing the O2 sensor.

Model-Specific Issues

If you are having trouble finding the cause of the code(s), a particular component specific to your vehicle model may be behind the problem. Consult your vehicle repair manual.

If you don't have the manual for your model, you can buy a relatively inexpensive aftermarket manual from Amazon. Haynes manuals can not only help you locate components in your vehicle, but come with step-by-step procedures for many maintenance, repair, and troubleshooting projects you can do at home. So you can save thousands of dollars a year on many simple maintenance tasks and repairs that you can do right at home.

OK. Let's take a look at the most common sources of trouble so you can diagnose the cause of the problem in your engine.

A MAF sensor with a dirty sensing element can throw a P0171 code.

A MAF sensor with a dirty sensing element can throw a P0171 code.

2. MAF Sensor and Throttle Body Issues

A common source of trouble code P0171 or P0174 is a dirty or failing MAF sensor. It can produce the following symptoms:

  • Increased fuel consumption
  • Engine surging
  • Rough idling
  • Lack of engine power
  • Engine hesitation or surging
  • Engine surging at idle

Dirt can enter the MAF sensor housing and obstruct the film element or hot wire.

Cleaning the MAF Sensor:

You can make a quick visual inspection of the MAF sensor by unplugging the sensor's electrical connector and disconnecting the air house assembly.

A dirty filament inside the sensor can interfere with the sensor's ability to measure incoming air. To clean the sensor:

  • Get a can of MAF sensor cleaner
  • Gain access to the sensor as suggested by your repair manual
  • Follow the instructions on the product's package to clean the sensor
  • If necessary, also replace the air filter and clean inside the filter's housing. Remove debris and dust using a damp rag. This will prevent contaminating the new or newly cleaned sensor and prolong its service life.

The MAF sensor's harness may be faulty as well. Check the connector and wires using a digital multimeter (DMM), if necessary.

Problems with the fuel system or vacuum leaks can also produce some of the same symptoms as well. If necessary, test the MAF sensor using a digital multimeter.

Checking the Throttle Body:

After checking your MAF sensor, check for carbon buildup around the bore of the throttle body and under the valve. This can also cause problems.

To Clean the Throttle Body:

  • Disconnect the air cleaner assembly from the throttle body.
  • Spray some carburetor cleaner on a clean shop rag.
  • Use the rag to clean around the throttle body and valve.

The problem with codes P0171 and P0174 is that, often, drivers assume that the oxygen sensor has failed.

3. Vacuum Leaks

Problems in the vacuum system frequently contribute to a lean condition as well.

  • Look for potential leaks around the air cleaner assembly, between the MAF sensor and the throttle body. A loose or damaged boot can introduce unmetered air.
  • To check for potential gaskets problems under the throttle body and intake manifold:

    • spray carburetor cleaner around the gasket sealing area with the engine at idle.
    • If idle speed changes as you spray a particular area, there could be a leak.

Other Key Areas to Check for a Vacuum Leak:

  • Check for loose vacuum hoses.
  • Look for damaged or cut vacuum hoses.
  • Inspect vacuum hoses routed near hot areas like the engine or exhaust system for possible damage.
  • Check vacuum-operated devices for damage that might be causing a leak.
  • Make sure each fuel injector is properly seated and their O-rings are not damaged.
  • Inspect the vacuum hose between intake manifold and charcoal canister purge valve.
  • Although not too common, an EGR valve can also be the source or contributor to a lean condition. Check for loose mounting bolts or a damaged gasket under the valve.
  • On some Ford models, a Delta Pressure Feedback EGR (DPFE) sensor may develop too much corrosion to the point of failure. The ECU (car's computer) ends up opening the EGR valve too long, causing a lean condition.
  • Inspect the exhaust system for leaks between the engine and the O2 sensor as well. Idle the engine and then increase engine speed slightly while paying attention to sounds or smoke coming from the gasket area, pipe connections, or anywhere between the engine and catalytic converter.

This other post on finding and repairing vacuum leaks goes into more detail and can be of great help if you suspect a leak.

Using a Vacuum Gauge:

If necessary, you can use a vacuum gauge to check the for potential leaks. Here's a simple procedure:

  1. Warm up the engine to operating temperature.
  2. Connect the vacuum gauge to the intake manifold.
  3. Set the parking brake and set the transmission to Park (automatic) or Neutral (manual).
  4. Start the engine and let it idle.
  5. Get the vacuum reading from the gauge.

    • An engine in good condition will produce a vacuum reading with the needle steady between 15 and 20 inHg (inches of mercury) at sea level. At higher elevations (2000 feet or more), subtract one inHg for every 1000 feet above sea level.
    • A leak in the intake manifold will show a steady needle at the lower end of the gauge.
    • A leaking head gasket may cause the needle to fluctuate between 6 and 20 inHg.
Check and replace the PCV valve, if necessary.

Check and replace the PCV valve, if necessary.

4. PCV System Issues

A stuck-open valve or torn hose in the positive crankcase ventilation (PCV) system can also lead to a lean air-fuel ratio and cause rough idle and stalling problems.

A leaking engine gasket can cause the PCV valve to draw unfiltered air into the intake.

Usually, a quick inspection can reveal the cause of the problem:

  • Check PCV hoses for proper connection and condition.

Many PCV valves use an internal check needle to control airflow in one direction:

  • Remove the valve from the valve cover and disconnect the hose from the valve.
  • Connect a hose to the outlet side of the valve (the end that points to the intake manifold).
  • Blow air through the hose.
  • If you can easily blow air through the valve, it is stuck open. Replace the valve.

This post on PCV valve tests goes into more detail about the procedure.

Replace the fuel filter if it is overdue.

Replace the fuel filter if it is overdue.

5. Fuel System Issues

Problems with the fuel system are also common causes of a lean air-fuel ratio.

If the engine seems to lack sufficient fuel at any speed range, the problem may be caused by:

  • Restricted fuel filter
  • Clogged fuel injector(s)
  • Bad fuel pump

Injector fuel restriction can be produced by high resistance in the injector's electrical circuit. When current can't flow properly through a wire or connector, it prevents an injector's pintle from fully opening, which results in less fuel being injected.

Or the engine may run well at idle but be starved of fuel at higher speeds or under load. There could be one or more reasons for this:

  • Clogged fuel filter
  • Failed fuel pump
  • Bad pressure regulator
  • Clogged or dirty fuel injectors or inlet screens
  • Fuel injector electrical circuit problems

The problem may be solved by simply replacing the fuel filter. Check your car owner's manual or vehicle repair manual for the filter recommended service schedule. If it's overdue, replace it and see if engine performance improves.

Checking the Fuel Delivery System:

On a throttle body injection (TBI) system, you can confirm fuel is being delivered by removing the lid from the air filter housing. While cranking the engine or idling the engine, you can visually inspect fuel injection and fuel pattern as it comes out the injector.

You should see an inverted V, partially atomized, fuel delivery pattern coming out of the injector. An irregular pattern may indicate a restricted or bad fuel injector, or insufficient fuel system pressure.

You may want to check fuel pressure using a fuel pressure gauge if you suspect insufficient fuel is being delivered. Consult your vehicle repair manual, if necessary.

Checking the fuel pressure and comparing it with the specs for your particular vehicle make and model will really help in your diagnostic, as you can see in the next video.

Checking Fuel System Volume:

Along with fuel system pressure, fuel system volume is another common fault in a lean-running condition.

In this case, the fuel system may or may not have adequate pressure but it may not deliver the correct fuel volume according to engine running conditions, especially at higher RPMs or at engine load.

You may suspect insufficient volume if everything else seems to be in order. In general, a fuel system will deliver about three pints of fuel for 30 seconds.

You need a graduated container and a fuel pressure gauge with a release valve to conduct a volume test. Still, you can get a rough idea of how your fuel system is working using a fuel pressure gauge to conduct this alternate test at home following the steps described in the next diagram.


Keep in mind that fuel pump volume may be somehow low and you may not notice the effects unless your engine is under heavy load. But the above test can give you a rough idea of fuel pump condition.

Checking Fuel System Pressure on Multiport Fuel Systems:

On multiport fuel injection systems, you can't check fuel delivery as easily as with a TBI system. So you need to test fuel pressure and, if necessary, fuel volume.

You'll need a fuel pressure gauge and a pair of slip-joint or rib-joint pliers for the following tests. During the tests, wrap a rag around any fuel hose you need to pinch with the pliers.

  1. Connect the fuel pressure gauge to the test port (Schrader valve) on the fuel rail. If your vehicle doesn't come with a test port, check your vehicle repair manual regarding how to connect the pressure gauge.
  2. Idle the engine for about 15 minutes to bring it up to operating temperature.
  3. Turn off the engine and pinch the fuel return line using the pliers.

    • If the correct system pressure remains, the leak could be in the pressure regulator.
    • If pressure drops, continue to the next step.
  4. Turn the ignition key to the "On" position to operate the fuel pump and build pressure in the system.
  5. Now, pinch the fuel supply line with the pliers before the pressure drops. Your fuel pressure gauge should be located between the blocked point and the fuel injectors.

    • If system pressure remains this time, the fuel pump check valve could be leaking.
    • If system pressure drops, the fuel rail or a fuel injector(s) could be leaking.

Many late-model vehicles use now a returnless fuel system type. Consult your vehicle repair manual to check this type of system.

This other post can help you test your fuel injectors, if necessary.

Although not as common, the oxygen sensor can fail and cause a P0171 code.

Although not as common, the oxygen sensor can fail and cause a P0171 code.

6. Oxygen Sensor Issues

If your computer has triggered a trouble code for a possible failed oxygen or AFR sensor along with other codes, check those other codes first. Many times, problems in other areas will affect the O2 sensor readings.

An exhaust leak can also influence the sensor's output.

But it's true that oxygen sensors can also fail. Testing can help you verify the sensor operating condition.

A quick way to test the sensor is to swap sensors if your vehicle uses more than one. If the problem persists in the same location, either the sensor is working fine, and the problem source is somewhere else: possibly a vacuum or exhaust leak; or there's a problem with the sensor's electrical circuit. Check the connector and wires, if necessary.

When checking an O2 sensor, visually inspect it for possible contamination. Then go ahead and go over this other post on oxygen sensor troubleshooting.

Additionally, you may want to check for technical service bulletins (TSB) for your particular vehicle make and model related to a lean-running condition.

High-mileage engines can suffer serious mechanical problems that can throw a P0171 or P0174 code among other issues.

High-mileage engines can suffer serious mechanical problems that can throw a P0171 or P0174 code among other issues.

7. Preventing Damage From Trouble Codes P0171 and P0174

The problems that cause trouble codes P0171 and P0174 can not only hurt engine performance but, as time goes by, damage other components. They can hurt the catalytic converter, for example; misfires allow raw fuel to enter the cat.

Also, if unmetered or unfiltered air is leaking through the intake manifold or air ducts, dust particles can eventually damage cylinder walls. This is an expensive repair you want to avoid.

A simple job like cleaning the MAF sensor or replacing a clogged fuel filter may solve your problem. But ignoring the problem can result in an expensive repair later on.

It's cheaper to clean a sensor or replace a filter than replace a catalytic converter and change other affected components.

So follow this guide and start checking the most common sources for trouble codes P0171 and P0174.

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: My 2000 Chevy Blazer gets both P0171 and P0174 codes. I've changed the MAF sensor, fuel pump, spark plugs, and the O2 sensor after the catalytic converter. I don't know what else to do. The check engine light comes on but will go off after a while. But while on, the engine runs rough. Once it goes off, smooth sailing. What should I do?

Answer: This seems to be an intermittent fault and can be hard to diagnose. The codes can be triggered by other faults. Common issues include dirty fuel injectors (loose connectors or damaged internal coils), leak in the PCV system, vacuum leaks (loose hoses), EGR system faults (bad solenoid connection). Check these other components as well. Hope this help.

Question: I've got P0171 error code. Lack of power in the lower rpms/sluggish. I recently replaced my cat with a pipe. Also, used a spacer on my bank 2 o2 sensor to trick it. Could this be the reason I got the code? Why would it come up for bank 1 if so?

Answer: Most likely the sensor is reading too much oxygen. The problem seems to be around the side with cylinder one. Just pretend you still have the cat in place and diagnose bank one. See how it goes.

Question: What is most likely cause if I am getting P0171 and P0174 error codes and the engine won't start?

Answer: Try spraying some carburetor cleaner through the throttle and see if the engine catches for a second or two when cranking. If it does, check for fuel pressure. Also, check for a vacuum leak (PCV system, EGR, or hose). Then check for spark.

Question: I get a P0174 code only sometimes . After my engine has been on for awhile, then shut it off, then restart and get on the freeway, then it will come on. What should I check to find the problem?

Answer: The problem seems to be affecting only bank two of the engine. Check for fuel injector problems or a vacuum leak or an oxygen sensor for that side. Bank one is the side of the engine with cylinder number one. Bank two is the opposite side.

Question: I have codes P0171, P0174, P0403, and P0443. Is that telling me it’s either the EGR valve and/or purge valve?

Answer: Check first the circuit for the EGR solenoid and take it from there.

Question: My BMW 328i xDrive has both p0171 and p0173 codes with the service engine soon indicator. Car runs smooth, no idling or misfiring issue. Normal throttle response. Checked visually for vacuum leaks and hissing noises. Nothing obvious. Since both cylinder banks showing an error, I am inclined to rule out O2 sensor issue. What could be other possible causes?

Answer: It could be a simple thing as a dirty air filter, or a stuck-open PCV valve. Check for a dirty MAF sensor and, possibly, a fault in the EGR system.

Question: Are vacuum hoses more likely to leak around manifold 2.3 liter?

Answer: Vacuum hoses are a common source of vacuum leaks. After years of service, they may become brittle, weak and collapse, tear and loose when working around the engine.

Question: I get both P0171 and P0174 codes at the same time only when the weather is below -25C. What is the significance of this in terms of the temperature, and the fact that I always get 171/174 together (vs only one or the other)?

Answer: The fact that you are getting both codes means that a lean condition is affecting both cylinder banks. Since you only get these codes at a certain temperature, it's probably an issue with a false sensor reading. For example, the intake air temperature sensor might be sending an incorrect signal at freezing temperature or the oxygen sensor itself or the circuit, if voltage in the circuit is being affected (dirty or loose engine grounds, perhaps plus high demand from the battery upon the alternator). As temperature drops, engine demands increases but battery output decreases.

Question: 2005 Dodge Dakota 3.7L with P0171 and P0174. I replaced four O2 sensors, plugs/coil packs and still stumbles. There's a rotten egg smell, what gives?

Answer: There could be a problem with the catalytic converter (rotten eggs smell). This other post may help:

Question: Why am I getting error codes PO171 PO302 PO304 PO306 PO300 PO174, are they likely all connected?

Answer: Look for possible faulty injectors and/or a vacuum leak.

© 2019 Dan Ferrell