Draft guidance: awaiting Claude spot-check, qualified-mechanic review and named-editor approval. Do not use a fault code alone to replace parts.
P0171: Fuel-trim system too lean, Bank 1
The computer has had to add more fuel than its normal correction range to counter a lean indication on Bank 1.
What P0171 means
The computer has had to add more fuel than its normal correction range to counter a lean indication on Bank 1. In practice, too much unmetered air, too little fuel or misleading sensor information can all cause it. P0171 does not automatically mean the oxygen sensor is faulty.
Can I still drive?
This is more urgent than an isolated downstream-sensor code. A mild case with a steady light may permit a gentle trip directly for diagnosis, but sustained lean running can cause misfire, knock and high combustion or catalyst temperatures. Stop for a flashing light, audible knock, severe hesitation, stalling or major power loss.
Symptoms
- Engine-management light
- Rough or surging idle
- Hesitation, weak acceleration or loss of power
- Difficult starting or stalling
- Misfire or metallic pinging/knock under load
Common causes
- Split intake duct, vacuum or crankcase-ventilation hose, or leaking intake gasket
- Incorrect mass-airflow reading
- Low fuel pressure, restricted filter, weak pump or restricted injector
- Exhaust leak before the upstream mixture sensor
- Biased oxygen or air/fuel sensor, leaking purge system, or less common mechanical/timing fault
Safe first checks
- Record every code plus freeze-frame and fuel-trim data. With the engine off, inspect intake boots, clamps, vacuum and crankcase-breather hoses, and the airflow-sensor connector. Check for visible soot at accessible exhaust joints only when cold. Do not spray flammable products around a running engine, open the fuel system, or clean an airflow sensor with an unsuitable solvent. A garage smoke test and fuel-pressure test are safer next steps.
Typical UK fix cost
About £90–£650 for diagnosis and many common repairs, from a hose or intake leak through airflow-sensor or moderate fuel-system work. A fuel-pump replacement alone may be roughly £300–£900, so the eventual bill can exceed the headline range. Insist on testing because P0171 has many possible causes.
Related models
This is a generic OBD-II code used across many makes and models. The exact test thresholds, component layout and repair route vary by engine and manufacturer, so check vehicle-specific technical information before ordering parts.
Related fault codes
- P0140: Oxygen sensor circuit no activity detected, Bank 1 Sensor 2
- P0141: Oxygen sensor heater circuit malfunction, Bank 1 Sensor 2
- P0172: System too rich (Bank 1)
- P0174: System too lean (Bank 2)
Sources
A diagnostic trouble code identifies a system or circuit to investigate. It does not prove that a named part has failed. Preserve freeze-frame data and seek a vehicle-specific diagnosis before clearing the code.