You read your car’s computer and see error code P0171, sometimes together with P0174. It sounds vague, but it is common and usually easy to find yourself. In this article, you will read what P0171 means, what the causes are, and how to fix it without unnecessarily replacing expensive parts.
What does error code P0171 mean?
P0171 stands for "System Too Lean (Bank 1)." In plain language: the fuel mixture on bank 1 is too lean. There is too much air or too little fuel. The engine computer first tries to correct this by adding more fuel. If that is not enough, it sets the error code and turns on the check engine light.
If you also get P0174, the same applies to bank 2. This usually points to a common cause, such as false air or a dirty mass air flow sensor.
Symptoms
Check engine light on. Sometimes the only noticeable sign. The code is then already stored.
Unstable idle or stalling. Especially when cold or at idle speed, the engine can run irregularly.
Power loss and higher consumption. The engine does not run optimally, which you notice in pulling power and fuel consumption.
Possible causes (from cheap to expensive)
False air / vacuum leak. The most common cause. A torn hose or leaking gasket lets unmeasured air in. Usually cheap to fix.
Dirty mass air flow sensor (MAF). A dirty MAF measures too little air. Cleaning with MAF cleaner often helps; otherwise replace.
Leaking or dirty injectors. Too little fuel due to clogged injectors can cause a lean mixture.
Weak fuel pump or dirty filter. Too low fuel pressure makes the mixture lean. Slightly more expensive to diagnose.
How to determine the cause
Use a scanner to read the fuel corrections (fuel trims). If the short and long term fuel trims are significantly positive (for example above 10 to 15 percent), that confirms a lean mixture. Then look for false air: spray some brake cleaner around the intake hoses and gaskets with the engine idling. If the RPM changes, you’ve found a leak. Also check the MAF values in the live data.
Which scanner do you use for this?
An affordable scanner is sufficient for reading and live data of the fuel trims:
View the Topdon CarPal → 
View the OBDeleven 3 → 
View the Launch CRP919X →
The Topdon CarPal (around €40) reads the code and fuel trims via your phone. The OBDeleven 3 (around €77) does that plus all modules. The Launch CRP919X (around €440) adds active tests.
Frequently Asked Questions
Is it okay to keep driving with P0171?
Usually you can drive for a short time, but a lean mixture can damage the engine and catalytic converter over time. So don’t ignore it for too long.
What is the difference between P0171 and P0174?
P0171 relates to bank 1, P0174 to bank 2. Together they often indicate a common cause like false air or a dirty mass air flow sensor.
Does cleaning the MAF help?
Often yes. A dirty mass air flow sensor is a common cause. Clean it carefully with special MAF cleaner, not brake cleaner.
Can I just clear P0171?
Clearing is possible, but if the cause remains, the code will return. First address the cause, then clear the code and check if the fuel trims are back to normal.
In short
P0171 means a lean mixture on bank 1. The cause is usually false air or a dirty mass air flow sensor, not necessarily an expensive part. Read the fuel trims and look for leaks before replacing anything. Check out the scanners in our diagnostic equipment collection or read how to read and clear fault codes.