P0171 — System Too Lean (Bank 1)
Moderate severityWhat it means
The engine control unit is adding the maximum allowed fuel correction because the air–fuel mixture on bank 1 is too lean — too much air or too little fuel. The most common culprit by far is an unmetered air leak after the MAF sensor.
Common symptoms
- Rough idle or hesitation
- Hard starting, especially when cold
- Lack of power and possible pinging
- Check engine light, sometimes with P0174
Likely causes
- Vacuum or intake leaks after the MAF sensor (most common)
- Dirty or faulty MAF sensor
- Weak fuel pump, clogged filter or injectors
- Faulty upstream oxygen sensor
- PCV system or purge valve leaks
What to check first
- Listen and look for intake/vacuum leaks (hoses, clamps, gaskets)
- Compare MAF readings (g/s at idle) against expected values in live data
- Watch long-term fuel trims — values far above +10% confirm a lean condition
- Test fuel pressure if no leak is found
Can you keep driving?
Drivable in the short term, but a lean mixture raises combustion temperatures and can cause damage over time. Diagnose it soon.
Frequently asked questions
What is the most common cause of P0171?
The most likely cause is: Vacuum or intake leaks after the MAF sensor (most common). Several other faults can set P0171 as well, so confirm the real cause with live data and the checks above before replacing any parts.
Can I just clear P0171 and keep driving?
Clearing P0171 with an OBD2 scanner turns the warning light off, but if the underlying fault has not been repaired the code will usually come back within a few drive cycles. Fix the cause first, then clear the code to confirm the repair worked.
How serious is P0171?
P0171 is a moderate-severity code. The car is often still drivable, but have it diagnosed soon — the underlying problem can get worse and it may fail an emissions test.
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