P0171OBD-II fault codes
OBD-II P0171 — System Too Lean (Bank 1)
The ECU is adding fuel beyond its limit because Bank 1 sees too much air.
Severity: mediumBank 1 — inline and V engines with OBD-IIToyota · VW · Ford · Renault · BMW
Technical meaning
Long-term fuel trim on bank 1 exceeds the positive limit — too much air or not enough fuel.
Symptoms
- Check-engine light with higher fuel consumption
- Rough or unstable idle
- Hesitation when accelerating
- Lean misfire under load
Most likely causes
- 1Unmetered air from a cracked intake hose or leaking PCV valve
- 2Dirty or failing mass air flow (MAF) sensor
- 3Weak fuel pump, clogged filter or leaking injectors
How to fix it
- 1Read freeze-frame data and check long-term fuel trim: values above +20 % confirm a lean condition.
- 2Inspect every intake hose after the MAF sensor for cracks and loose clamps.
- 3Clean the MAF sensor with dedicated MAF cleaner — never touch the wire.
- 4Check fuel pressure at the rail and the condition of the filter.
- 5Smoke-test the intake and vacuum lines if no visual leak is found.
- 6After the repair clear the code and verify that fuel trim returns to ±10 %.
ELM327 / OBD-II
ATZ # ELM327 reset ATE0 0105 # coolant temperature 010B # intake manifold pressure 0110 # MAF air flow rate 0101 # fuel trim / DTC count 03 # read stored DTCs 04 # clear DTCs after the repair
Frequently asked questions
- Is P0171 dangerous for the engine?
- Driving long-term with a lean mixture raises combustion temperature and can damage valves and the catalytic converter.
- Can a bad oxygen sensor cause P0171?
- Rarely. A lazy O2 sensor usually reports its own faults — check air leaks and fuel delivery first.
Copies: 31,540 · Views: 96,120 · Updated: Sep 15, 2026
Related entries
The ECU detected misfires on several cylinders without a single identifiable source.
errors
The boiler tried to ignite but the flame sensor never confirmed a flame.
errors
The downstream oxygen sensor sees too little catalyst work on bank 1.
errors