Vacuum leak from a cracked or disconnected intake hose, intake manifold gasket, or PCV hose; Contaminated or failing mass airflow (MAF) sensor sending inaccurate air readings; Weak or failing fuel pump delivering insufficient volume under demand; Clogged fuel filter restricting fuel flow during acceleration; Leaking intake manifold gasket allowing unmetered air into the engine; Faulty oxygen sensor (O2 sensor) providing incorrect feedback to the PCM; Exhaust leak before the oxygen sensor causing false lean readings

DTC codes / GM codes / P1171

P1171 · GMFuel System Lean During Acceleration

Q: What does code P1171 mean?

Fuel System Lean During Acceleration. P1171 on GM vehicles means the powertrain control module (PCM) has detected a lean fuel condition specifically during acceleration, where the air-fuel mixture contains too much air and not enough fuel. The PCM monitors oxygen sensor feedback and fuel trim corrections, and when it must add excessive fuel to maintain the target ratio under acceleration, it stores this code.

P1171 on GM vehicles means the powertrain control module (PCM) has detected a lean fuel condition specifically during acceleration, where the air-fuel mixture contains too much air and not enough fuel. The PCM monitors oxygen sensor feedback and fuel trim corrections, and when it must add excessive fuel to maintain the target ratio under acceleration, it stores this code.

SystemPowertrain

Code typeGM specific

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Check engine light illuminated, often with no other noticeable symptoms
Hesitation or stumble when accelerating from a stop or during passing
Reduced engine power or sluggish response under load
Occasional rough idle or misfire that worsens during acceleration
Poor fuel economy despite the lean condition
Engine may surge or feel like it is running out of fuel at higher speeds

Common causes, most likely first

Vacuum leak from a cracked or disconnected intake hose, intake manifold gasket, or PCV hose
Contaminated or failing mass airflow (MAF) sensor sending inaccurate air readings
Weak or failing fuel pump delivering insufficient volume under demand
Clogged fuel filter restricting fuel flow during acceleration
Leaking intake manifold gasket allowing unmetered air into the engine
Faulty oxygen sensor (O2 sensor) providing incorrect feedback to the PCM
Exhaust leak before the oxygen sensor causing false lean readings

How to diagnose and fix it

Inspect all intake and vacuum hoses for cracks, loose clamps, or disconnected lines and repair as needed
Clean the mass airflow sensor with a dedicated MAF cleaner and verify its readings with a scan tool
Check fuel pressure and volume under load using a fuel pressure gauge; compare to GM specifications
Replace the fuel filter if it is restricted or has not been serviced at the recommended interval
Perform a smoke test on the intake system to locate hidden vacuum leaks
Test the oxygen sensors for proper switching and replace any that are slow or biased
Inspect the exhaust system for leaks upstream of the oxygen sensors and repair any found

What most technicians miss on P1171

A dirty mass airflow sensor often causes this code without setting a MAF-specific trouble code, so cleaning it is a smart first step before replacing expensive parts.
Fuel pump volume can be within specification at idle but drop off under acceleration, so a static fuel pressure test alone may not reveal the problem.
Intake manifold gasket leaks on GM V6 and V8 engines are common and can be intermittent, only showing up when the engine is cold or under certain load conditions.
An exhaust leak before the oxygen sensor can draw in outside air and trick the PCM into thinking the mixture is lean, leading to unnecessary fuel system repairs.
This code does not always mean a failed component; a loose or improperly seated oil dipstick or oil filler cap can create a vacuum leak on some GM engines.

Can I keep driving with P1171?

You can drive the vehicle short distances to a repair shop, but avoid hard acceleration and heavy loads because the lean condition can cause overheating and potential engine damage. If the check engine light begins flashing, stop driving immediately and have the vehicle towed.

Tools that help

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Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.