Fault-Code Library · Fuel mixture
P0171: System too lean, bank 1
Fuel trim has reached a lean threshold because the ECU is adding more fuel than expected.
Common symptoms
- •Rough idle
- •Hesitation
- •Whistling noise
- •Misfires
Likely causes
- •Vacuum or PCV leak
- •Low fuel pressure
- •Incorrect airflow reading
- •Exhaust leak before the oxygen sensor
First checks
- •Review short- and long-term fuel trims
- •Smoke-test the intake
- •Check PCV operation and fuel pressure
Important
Use trim behavior at idle and under load to separate air leaks from fuel-delivery faults.
A fault code identifies a detected condition, not a failed part. Confirm manufacturer-specific subcodes and live data.