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.
Scan, log and diagnose with MechanicAI
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- And above all — analyze it all with AI for the most likely cause and clear next steps.
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