P0171
OBD-II code guideSystem too lean on bank 1
Understand the P0171 system-too-lean code, its common vacuum and fuel causes, severity, and the practical next inspection step.
Plain-English meaning
What the signal is telling you
The engine computer is adding more fuel than expected because bank 1 appears to have too much air or too little fuel. The code describes the correction the computer is making, not the failed component.
Typical severity
How quickly to act
Moderate — drivability can worsen
Severity depends on how the vehicle is behaving and which other codes are present.
Likely causes
These are starting points for inspection, not an automatic parts list.
- A vacuum leak or split intake hose after the mass-airflow sensor
- A dirty or inaccurate mass-airflow sensor
- Low fuel pressure, a restricted filter, or a weak injector
- An exhaust leak ahead of the bank 1 oxygen sensor
Possible symptoms
Symptoms vary by vehicle and may have more than one cause.
- Rough or uneven idling.
- Hesitation or reduced power when accelerating.
- Hard starting, stalling, or worse fuel economy.
Practical next steps
Give the technician useful context before replacing anything.
- 1Note whether the roughness is worse at idle or under load; that helps separate air and fuel faults.
- 2Have the intake tract smoke-tested and fuel trims checked before parts are replaced.
- 3Stop driving and arrange an inspection if the engine is stalling, overheating, or flashing the warning light.
Have this code on your dash?
Turn the signal into a useful report.
Add your vehicle and symptoms so PitPilot can help identify the right first inspection.