OBD-II · Engine and powertrain (P)
P0171
System too lean bank 1
What it means
Bank 1 is running with too much air or not enough fuel.
Symptoms
- engine warning light
- hesitation or flat spots when accelerating
- rough or hunting idle, sometimes stalling
- may be worse when cold or at idle
Likely causes, most likely first
| No. | Cause |
|---|---|
| 01 | vacuum leak, often a failed crankcase breather (PCV) valve or split intake hose |
| 02 | dirty or failing MAF sensor under-reading air flow |
| 03 | low fuel pressure (weak pump, clogged filter) |
| 04 | exhaust leak before the lambda sensor |
| 05 | LPG system running lean on converted cars |
What to check, step by step
- 01Read short- and long-term fuel trims at idle and at about 2,500 rpm: high trims at idle that improve with rpm point to a vacuum leak; high trims at all speeds point to fuel delivery or the MAF
- 02Smoke-test the intake and check the crankcase breather/PCV valve (a whistle or strong suction at the oil filler cap is a clue)
- 03Compare MAF readings with OEM values; clean only if permitted by the sensor maker, otherwise replace a confirmed faulty sensor.
- 04Check fuel pressure and delivery (pump, filter)
- 05On LPG-converted cars, check whether the trims are normal on petrol; if so, service or recalibrate the gas system
- 06Check for exhaust leaks before the upstream lambda sensor