Engine and gearbox (P)
P0171: System Running Lean
The engine control unit has detected that the air-fuel mixture is too lean, meaning there is too much air and not enough fuel. This can lead to poor engine performance and increased emissions.
Checked by a licensed car technician · last reviewed 29 September 2026
Symptoms
- Rough idle
- Poor acceleration
- Increased fuel consumption
- Engine misfires
- Check engine light on
Common causes
- Vacuum leak
- Weak fuel pump
- Dirty mass air flow sensor
- Faulty oxygen sensor
- Clogged fuel filter
How to check it yourself
- Inspect intake hoses and connections for leaks
- Check fuel pressure
- Clean or replace mass air flow sensor
- Inspect and test oxygen sensors
- Replace fuel filter if clogged
Can I keep driving?
The car can be driven, but prolonged driving with this issue may cause engine damage.
General guidance, not a professional inspection. Confirm faults with measurements before buying parts, and use a qualified mechanic for brakes, steering, fuel leaks, airbags, and other safety-critical work. Terms and disclaimer