Fuel / Air · Severity: Moderate
P0171 — System Too Lean (Bank 1)
Fuel trims on bank 1 have reached the lean limit — the engine is getting too much air or too little fuel.
Common symptoms
- Rough idle
- Hesitation
- Misfires
- Engine light
Likely causes
- Vacuum leak
- Dirty or faulty MAF sensor
- Weak fuel pump or clogged filter
- Leaking intake gasket or PCV
First test to run
Compare short and long-term fuel trims at idle and at 2,500 rpm; if trims drop at higher rpm, smoke-test for vacuum leaks.
Diagnose P0171 on your vehicle
Pick the exact car and get a step-by-step plan: tests, real access times, tools and the repair — free for one car.