P0171Fuel and air

P0171: System Too Lean Bank 1

P0171 means the engine computer detects too much air or too little fuel on bank 1 (the side of the engine with cylinder 1) and has already added the maximum extra fuel without fixing it. You can usually drive for a short time, but the cause should be found soon because the engine may run rough and the code affects emissions.

You can drive, but get it checked soon

The data lists no limp mode for this code, so short trips are usually possible. A lean mixture can cause rough idle, stalling and hesitation, and the cause should be repaired soon rather than ignored.

What it means

The engine computer constantly adjusts how much fuel is injected, using the oxygen sensor in the exhaust to check the result. Short-term fuel trim reacts within fractions of a second, while long-term fuel trim is a learned base correction. Positive trim values mean fuel is being added because the mixture was lean, and about ±10 % is normal. P0171 is stored when long-term trim on bank 1 reaches its upper limit: the computer is already adding the maximum fuel and the mixture still stays lean. The code needs two trips to set, so the check engine light may not come on right away. If P0174 (the same fault on bank 2) appears together with P0171, a shared cause is likely. The oxygen sensor only reports what is in the exhaust and is rarely the cause.

Likely causes

Air leak after the air flow sensor (intake manifold gasket, vacuum hoses, throttle body gasket, brake booster line)Very likely
Air that enters after the sensor is not measured, so the computer does not add fuel for it and the mixture goes lean.
Crankcase ventilation leaking or faulty (PCV hose, diaphragm in valve cover, oil cap, dipstick)Very likely
A leaking crankcase ventilation hose or part lets unmetered air into the engine; GM bulletins repeatedly trace this code to leaking hoses or pipes on V6 and V8 engines.
Air flow sensor dirty or driftingPossible
A sensor that measures too little air makes the computer under-fuel the engine.
Fuel pressure or delivery too low (weak pump, clogged filter, regulator, tank nearly empty)Possible
If not enough fuel reaches the injectors, the computer cannot make up the shortage even at maximum correction.
Clogged injectors or an exhaust leak ahead of the oxygen sensorLess likely
Injectors that deliver too little fuel cause a lean mixture, and an exhaust leak can let outside air reach the sensor and make it read lean. Ford found leaking direct injectors as a cause on some engines.

What you may notice

Check engine light on (after two drive cycles)
Rough or unsteady idle, possibly stalling
Hesitation or lack of power when accelerating
Hissing or whistling from the engine bay (air leak, PCV)

What to check, cheapest first

1Check that the oil cap and dipstick are seated, and look over the crankcase ventilation (PCV) hoses for cracks or loose ends. · you can do it
2Listen at idle for hissing or whistling and inspect vacuum hoses, the brake booster line and the intake duct for cracks or loose clamps. · you can do it
3Make sure the fuel tank is not nearly empty, then clear the code and see whether it returns. · you can do it
4Read the fuel trim values with a scan tool. Very high trims only at idle that look normal under load point to an air leak; trims that rise with load point to a fuel shortage or an air flow sensor reading low. · mechanic
5Smoke-test the intake and crankcase ventilation system to find leaks at the manifold gasket, throttle body gasket and valve cover diaphragm. · mechanic
6Measure fuel pressure and check the air flow sensor reading and the injectors if no air leak is found. · mechanic

Frequently asked questions

Can I keep driving with P0171?+

Short trips are usually possible, but a lean engine can idle roughly, hesitate or stall. Have it diagnosed soon.

Can a loose gas cap cause P0171?+

The data does not list the gas cap as a cause. P0171 comes from extra air or too little fuel, so check air leaks, crankcase ventilation and fuel delivery first.

Will P0171 pass an emissions test?+

Not with the check engine light on. The code is emissions-relevant, so it should be fixed and the drive cycles completed before testing.

What does it mean if P0174 appears too?+

Both banks are running lean, which points to a shared cause such as an air leak before the engine, an air flow sensor problem or low fuel pressure.

Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10