When your vehicle’s powertrain control module (PCM) detects your vehicle’s air-to-fuel ratio is too lean, a P0171 code will be set. A lean air-to-fuel ratio means that the PCM is indicating that your engine is receiving too much air into the combustion chamber and not enough fuel.
The PCM receives signals from the oxygen sensors within your engine bay and is always working to adjust the proper amount of air and fuel to the engine based on these readings.
When the factory defined range is out of whack, or more specifically lower in fuel than the engine needs, this will trigger the P0171 code. The PCM then asks for more fuel to be sent through the fuel injectors into the combustion chamber of the engine.
This is called a fuel trim adjustment, and in the case of a P0171 where more fuel is needed, this is called a positive fuel trim adjustment.
What Does the P0171 Trouble Code Mean?
The P0171 code means “System Too Lean (Bank 1).” Bank 1 is the side of the engine containing cylinder 1, and “too lean” means the air-fuel mixture has too much air and not enough fuel.
The ECM constantly tracks this balance with data from the oxygen sensors in the exhaust system. If the sensors detect that the air-fuel ratio has remained too far from the ideal ratio of about 14.7:1 for an extended period of time, the ECM activates the P0171 code.
Why Does a P0171 Code and Running “Too Lean” Matter?
“Lean” running cars and light trucks are very polluting. Most of the NOx pollution, which is poisonous and can lead to asthma, is due to vehicles running too lean.
A lean running car can misfire, which sends raw fuel (HCs) to the catalytic converter causing possible internal damage and also putting it into the atmosphere. It hurts to be behind that car or truck misfiring, due to your eyes burning.
With a “rich” running engine (one not misfiring), it has no smell (CO has no smell), or you may smell rotten eggs which is sulfur dioxide from the Catalytic Converter.
P0171 is NOT an Oxygen Sensor problem. When a P0171 code is possible, the computer has already run a sequence of tests to prove out the oxygen sensor readings.
The oxygen sensors successfully passed the readiness tests and did not set any codes, prior to the computer looking to the Fuel Trim adjustment. When the computer determined the air-to-fuel mixture too lean it set the P0171 code.
Symptoms of the P0171 Code
If your car exhibits one or more of these symptoms, the P0171 code may have been set:
- Check Engine Light (CEL): The most obvious sign is the check engine light illuminated on the dashboard.
- Reduced Engine Performance: You may notice a lack of power, especially during acceleration.
- Rough Idle: The engine may idle unevenly or feel shaky.
- Stalling: In severe instances, the engine may stall, especially at low speeds or when coming to a stop.
- Increased Fuel Consumption: Ironically, regardless of the lean condition, the ECM overcompensates by injecting more fuel, causing poor fuel economy.
- Engine Hesitation: The vehicle may hesitate or jerk during acceleration.
Common Causes of the P0171 Code
A lean condition can occur for a variety of reasons, and identifying the ultimate reason will help to fix it properly. Here are the most common factors:
- Vacuum Leaks: Cracked or loose hoses, or gaskets from the intake manifold or the throttle body or EGR valve leaking.
- Defective Mass Air Flow (MAF) Sensor: If the MAF sensor is dirty or defective, it will send a wrong measurement to the ECM which produces an incorrect air-fuel mixture.
- Clogged or Dirty Fuel Injectors: A blocked fuel injector will reduce the amount of fuel reaching the engine.
- Weak Fuel Pump/Clogged Fuel Filter: If the fuel pump is weak, or if the fuel filter is plugged, it may cause a decrease in fuel pressure and cause a lean condition.
- Faulty Oxygen Sensor: A defective upstream oxygen sensor may report a wrong value to the ECM.
- Exhaust Leaks: A leak in the exhaust system near the rear oxygen sensor can result in false lean readings.
- PCV Issues: A bad Positive Crankcase Ventilation (PCV) valve can allow unmeasured air into the intake manifold.
- Software Issues: In rare cases, the software loaded into the ECM may not be properly interpreting the data from the sensor if, for example, the program is old or corrupt.
Diagnosing the P0171 Code
Before you reach a conclusion about the cause of the problem, it is essential to get an OBD-II scan done on your vehicle. You or a mechanic should scan for Code P0171 to know for sure that this is an accurate diagnosis for your vehicle and that there are no other fault codes to read upon viewing a check engine light.
Changing out some sensors might seem like it would fix it, but the OBD-II scan will verify that the issue lies somewhere around Bank 1.
You will want a friend who also works with diagnostics to check for Code P0171. An expert mechanic should have a little bit of additional equipment to really figure out what the problem truly is.
Following is a list of places to check after you have gotten Code P0171. Complete each of the steps below in the order they are listed until you discover what is causing the error message.
- Attach a vacuum gauge to find leaks —only when the engine is off.
- Connect a fuel pressure gauge to check fuel sensor and mass flow air sensor functioning.
- Test oxygen sensors following the manufacturer instructions throughout the fuel system.
- Once every other option has been attempted, you will have identified the power control module likely being the issue.
Fixing the P0171 Code
The fix for a P0171 code depends on the underlying cause. Here are some common solutions:
- Vacuum Leaks repair: Install a new vacuum hose, or replace the faulty intake manifold gaskets. Tighten the screws and bolts that hold the vacuum connection together.
- Clean/replace the MAF Sensor: If cleaning the MAF sensor does not solve the problems, try a new one.
- Fuel System: Clean or replace the fuel injectors that are clogged, replace the fuel filter, or repair the fuel pump.
- Oxygen Sensors: replace the oxygen sensor if it is providing erroneous readings.
- Exhaust Leaks: If there are any exhaust leaks attached to or near the oxygen sensor, fix them.
- PCV Valve: If the existing PCV valve is faulty, put on a new one.
- ECM Software: If you believe the software is faulty, consult your dealership or mechanic about getting the software replaced.
Preventing the P0171 Code
Regular maintenance will help you easily prevent the P0171 code or other engine issues altogether. Here are a few things you can do to prevent the code:
- Clean Sensors: Clean or replace the MAF sensor and oxygen sensors as advised by the vehicle’s manufacturers.
- Check Vacuum Hoses: This should be done on a consistent basis as the hoses can wear down over time.
- Maintain the Fuel System: Use good fuel quality. Using fuel system cleaners from time to time lubricates, cleans and helps prevent clogs.
- Replace Air Filter: This is to ensure the engine has good air flow.
- Maintain Performance: Watch for changes in engine performance and remedy them sooner than later.
What is the cost to resolve a P0171 code?
The cost of fixing a P0171 trouble code can vary greatly depending on the cause. Repair estimates could include:
- Vacuum leak: $100–$200
- Mass airflow sensor (MAF): $300
- Fuel pump: $1300–$1700
- Fuel pressure regulator: $200–$400
- Oxygen sensor: $200–$300
- Exhaust repair: $100–$200
In addition to the actual repair, obtaining a diagnosis will be an additional cost of around $100, depending on how long it takes shop to find the issue. Because there is a long list of reasons why an engine will run lean, the average repair price will vary greatly.
However, you may get lucky with one of the simple repairs listed above. A broken vacuum hose sucking in air only costs $50 and a clogged fuel filter is $100.
On the other hand, the bill for an oxygen sensor or mass air flow sensor is around $400 to $500. Replacing a bad fuel pump is really expensive and could cost $600 to $800.
If you vehicle is throwing this code, make an appointment with a mechanic for a diagnostic check.