OBD-II code guide
P0171 System Too Lean Bank 1
P0171 means the engine computer is adding fuel because it sees too much air or not enough fuel on bank 1.
What it means
The air-fuel mixture is lean enough that fuel trims have exceeded the normal correction range.
Common symptoms
- Check engine light
- Rough idle
- Hesitation
- Hard starting
- Whistling or hissing from intake
Likely causes
- Vacuum leak
- Dirty mass airflow sensor
- Weak fuel pump
- Exhaust leak before oxygen sensor
- Low fuel pressure
First checks
- Inspect intake hoses after the air filter.
- Listen for vacuum leaks.
- Check fuel trim data if available.
- Avoid replacing oxygen sensors without testing.