Parts
$20–400
P0171 — System Too Lean (Bank 1)
CAUTIONIs it safe to drive? The engine is running lean — too much air for the fuel. Driveable, but it stresses the engine and can lead to misfires and a cooked cat over time.
What P0171 means
The computer is adding as much fuel as it can and the mixture is still lean. On a Bank-1 engine that's the side with cylinder 1. The classic tell: high positive long-term fuel trim.
Most likely causes (in order)
- Vacuum / intake leak (the #1 cause)
- Dirty MAF sensor under-reporting airflow
- Weak fuel pump or clogged fuel filter
- Leaking intake gasket or cracked vacuum hose
- Stuck-open EVAP purge valve
Symptoms you might notice
- Rough or hunting idle
- Hesitation / lean stumble
- Slightly worse fuel economy
What to check first
Watch long-term fuel trim: high positive AT IDLE that clears up by ~2,500 RPM points to a vacuum leak; trim that gets WORSE under load points to fuel delivery. Smoke-test the intake; clean the MAF and re-check g/s.
Repair cost & difficulty
Labor
0.5–3 hr
Difficulty
DIY-friendly