What P0171 means
The engine is receiving more air than the ECU accounts for, or less fuel than it commanded, and fuel trims have hit their correction limit.
Common causes
- Vacuum leak — intake gasket, PCV hose, brake booster line
- Dirty or failing mass airflow sensor
- Weak fuel pump or clogged fuel filter
- Leaking exhaust ahead of the upstream oxygen sensor
- Dirty fuel injectors
How it is usually fixed
- 1Smoke-test the intake system for leaks — the most common cause by far
- 2Clean the MAF sensor
- 3Check fuel pressure against specification
- 4Try a PEA fuel system cleaner if injectors are suspect
Other common codes
- P0011 — Camshaft Position A – Timing Over-Advanced (Bank 1)
- P0030 — HO2S Heater Control Circuit (Bank 1, Sensor 1)
- P0101 — Mass Air Flow Circuit Range/Performance
- P0113 — Intake Air Temperature Sensor Circuit High
- P0118 — Engine Coolant Temperature Sensor Circuit High
- P0131 — O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)
Code information is general guidance, not a diagnosis for a specific vehicle. Confirm with live data and manufacturer test procedures before replacing parts.