Pxxxx · Powertrain
P0171
System Too Lean (Bank 1)
ModeratePowertrain ECU
What it means
Fuel trims indicate the engine is running lean on Bank 1 — too much air relative to fuel. Common on aging vehicles with vacuum leaks.
Common causes
Listed in rough order of frequency. Verify with vehicle-specific service info before parts replacement.
- 01Vacuum leak (intake manifold gasket, PCV, brake booster hose)
- 02Failed MAF or MAP sensor reading low airflow
- 03Weak fuel pump or clogged fuel filter
- 04Failed oxygen sensor (upstream)
- 05Exhaust leak before the upstream O2 sensor
Affected systems
ECUs and subsystems that report or are affected by this code.
Engine Control Module (ECM)Fuel system
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