Severity: Moderate
Drivable, but it can cause more damage or poor economy. Get it checked within days.
What P0171 means
Bank 1 is running lean: the computer is adding a lot of fuel to correct for more air (or less fuel) than it expects. Long-term fuel trim is past its limit.
Symptoms
- Rough or high idle
- Hesitation on acceleration
- Possible misfire codes
- Poor fuel economy
Common causes
Most likely first
- 1Vacuum leak (intake gasket, PCV hose, brake booster hose)
- 2Dirty or failing mass airflow sensor
- 3Weak fuel pump or clogged filter
- 4Leaking intake air duct after the MAF
- 5Exhaust leak before the upstream O2 sensor
How we diagnose it
- Check short and long-term fuel trims at idle and 2,500 rpm
- Smoke test the intake
- Compare MAF reading to expected airflow
- Test fuel pressure and volume
Every vehicle sets codes under its own conditions. We check the factory diagnostic steps and service bulletins for your VIN, so the repair targets the actual cause.