What does P0171 mean?
P0171 means the engine control system has detected that the air-fuel mixture is too lean for the expected operating conditions.
In simple terms, the engine is detecting too much air or not enough fuel compared with the target mixture.
Common symptoms
- Rough idle
- Hesitation
- Poor acceleration
- Engine misfire
- Increased fuel consumption in some conditions
- Check engine light
- Hard starting in some cases
Common causes of P0171
- Vacuum leak
- Intake air leak
- Dirty or faulty mass-airflow sensor
- Fuel-pressure problem
- Restricted fuel supply
- Weak fuel pump
- Fuel injector problem
- Exhaust leak affecting oxygen-sensor readings
- Unmetered air entering the engine
How technicians diagnose P0171
A technician will normally look at fuel-trim data and compare short-term and long-term fuel corrections under different engine operating conditions.
The diagnosis may include inspecting intake hoses, vacuum lines, airflow sensors, fuel pressure, injectors, exhaust leaks, and related wiring.
Do not replace the oxygen sensor immediately
P0171 does not automatically mean that an oxygen or air-fuel sensor has failed.
The sensor may simply be reporting a real lean condition caused by an air leak or fuel-delivery problem.