P0171 System too lean (bank 1)
What P0171 means
The engine computer adjusts fuel delivery using feedback from the oxygen sensor. P0171 means it has had to add so much extra fuel on bank 1 that it has reached its correction limit. Either unmeasured air is entering the engine, or not enough fuel is being delivered.
P0171 is a generic SAE J2012 code: it means the same on every make. The exact cause still depends on the car, so read the freeze-frame data before replacing parts.
At a glance
- Severity
- High
- Typical repair cost
- 50–600 €
- Workshop time
- 0.5–4 h
- Repair difficulty
- Medium
- DIY possible
- Yes
- Check engine light
- Yes
- Affects emissions test
- Yes
Parts and labour at an independent workshop in Europe. Main dealers charge more; the cause decides the bill, not the code.
Symptoms
- Check engine light on
- Rough or unstable idle
- Hesitation when accelerating
- Possible misfires
Common causes
- Vacuum leak (cracked intake hose, intake gasket, PCV valve or hose)
- Dirty or faulty air-flow (MAF) sensor
- Weak fuel pump or clogged fuel filter
- Clogged injectors
- Exhaust leak before the front oxygen sensor
Can I keep driving with P0171?
Have it checked within a few days. Running lean for a long time can cause misfires, overheat the catalytic converter and, in severe cases, damage valves.
What to check first
- Check fuel trims at idle and at higher rpm: high at idle points to a vacuum leak, high at speed points to fuel supply or MAF
- Look and listen for vacuum leaks, check the PCV system and intake hoses
- Clean or test the MAF sensor
- Check fuel pressure and the front oxygen sensor
Related codes
- P0170 Fuel trim malfunction (bank 1)
- P0172 System too rich (bank 1)
- P0173 Fuel trim malfunction (bank 2)
- P0174 System too lean (bank 2)
- P0175 System too rich (bank 2)
- P0161 Oxygen sensor heater circuit (bank 2, sensor 2)
- P0181 Fuel temperature sensor A range/performance
- P0160 Oxygen sensor circuit no activity detected (bank 2, sensor 2)
Data: TORKO fault-code catalogue, built on OBDex (CC0-1.0) and reviewed against workshop practice. Informational only — it does not replace a diagnosis by a mechanic.
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