P0171: System Too Lean (Bank 1)
The engine on bank 1 is getting too much air or too little fuel, and the fuel trim has reached its limit.
Normal risk
Safe for short trips. Long-term lean running can cause misfires and overheating. Have it checked soon.
Short answer · Can I keep driving with P0171?
For short trips, yes. The engine may idle roughly and hesitate. Lean running for a long time can lead to misfires and engine damage, so have it checked soon. Vacuum leaks are the most common cause.
What does P0171 mean?
The engine control unit adjusts the fuel quantity using the front oxygen sensor ("fuel trim"). Code P0171 means it has added the maximum fuel it is allowed to on bank 1 and the mixture is still lean.
This means extra air is getting in, or too little fuel is arriving.
What the car does when the code is set
The check engine light comes on. The engine keeps running but its fuel correction is at its limit, so it may idle unevenly, hesitate when you accelerate and, in some cases, misfire. Fuel economy may change, and the engine may also store misfire codes if the mixture becomes too lean.
How the system works
The engine control unit calculates the fuel quantity from the air measured by the air-flow or manifold pressure sensor. The front oxygen sensor then reports whether the burnt mixture was rich or lean, and the control unit corrects the fuel quantity. This correction is called fuel trim: short-term trim reacts instantly, long-term trim learns a lasting offset. Both are limited. When the correction reaches its upper limit and the mixture is still lean, the engine sets a lean code, because something outside the control unit's measurement is adding air or removing fuel.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Vacuum leakA split hose, intake gasket or PCV valve lets in air the air-flow sensor does not measure.
- Dirty or faulty air-flow sensorThe sensor under-reports the air, so the engine adds too little fuel.
- Low fuel pressureA weak pump, clogged filter or faulty pressure regulator supplies too little fuel.
- Clogged injectorsDeposits reduce the amount of fuel injected.
- Exhaust leak before the front oxygen sensorOutside air makes the sensor read lean.
How it is diagnosed
- Look at the fuel trimsA scan tool shows short- and long-term fuel trims. Trims that are high at idle but fall at higher revs suggest a vacuum leak; high everywhere suggest fuel delivery or the air-flow sensor.
- Smoke-test the intakeA smoke machine shows leaks in hoses, gaskets and the PCV system quickly.
- Check the air-flow sensor readingCompare the measured air flow at idle and at higher revs with the value the service data gives for your engine.
- Measure fuel pressureA pressure gauge or the live data on direct-injection engines shows whether the fuel supply keeps up.
Questions about P0171
Why do P0171 and P0174 appear together?
When both banks are lean, the cause is usually shared: an air-flow sensor reading, a vacuum leak at a common point, or fuel pressure.
Can I just clean the air-flow sensor?
If the sensor is dirty, cleaning it with a dedicated sensor cleaner can help. Check for vacuum leaks first, as they are the more common cause.
Can a lean code damage the engine?
Running lean for a long time raises combustion temperatures and can cause misfires and, in extreme cases, burnt valves or pistons.