P0171: Fuel Trim System Too Lean (Bank 1)
MediumPowertrainFuel System
Symptoms
- Rough idle and stalling
- Hesitation under acceleration
- Lack of power
- Possible misfires at idle
Common causes
- Vacuum leak in intake hose or gasket
- Dirty or failing MAF sensor
- Weak fuel pump or clogged fuel filter
- Leaking or restricted fuel injector
- Bad oxygen sensor reporting wrong values
What to do?
Inspect the intake plumbing for cracked hoses and a torn intake boot - vacuum leaks are the most common cause. Clean the MAF sensor with proper MAF cleaner, then verify fuel pressure under load. Live data showing both short and long-term fuel trim above +10% confirms the lean condition.