Vacuum leak from a cracked or disconnected intake hose, intake manifold gasket, or vacuum line; Dirty or failing mass air flow (MAF) sensor reporting less air than actually enters the engine; Weak fuel pump or restricted fuel filter reducing fuel delivery; An exhaust leak ahead of the upstream oxygen sensor that lets outside air reach it and skew the reading; Faulty oxygen sensor (upstream) sending a false lean signal; Dirty or partly clogged fuel injectors on bank 1 that deliver less fuel than commanded; PCV valve or hose leak allowing unmetered air into the intake

DTC codes / P01xx · Fuel and air metering / P0171

P0171System Too Lean (Bank 1)

Q: What does code P0171 mean?

System Too Lean (Bank 1). P0171 means the engine computer has detected that the air-fuel mixture on bank 1 is too lean, meaning there is too much air or not enough fuel for the conditions it is measuring. The computer uses oxygen sensor feedback and fuel trim calculations to determine that it has reached the limit of adding fuel and can no longer correct the imbalance.

P0171 means the engine computer has detected that the air-fuel mixture on bank 1 is too lean, meaning there is too much air or not enough fuel for the conditions it is measuring. The computer uses oxygen sensor feedback and fuel trim calculations to determine that it has reached the limit of adding fuel and can no longer correct the imbalance.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Often no symptoms besides the warning light, especially in early stages
Rough idle or stumbling when the engine is cold or under load
Hesitation or lack of power during acceleration
Occasional misfire or engine surge at steady speed
Poor fuel economy despite the lean condition
Hard starting or stalling shortly after startup

Common causes, most likely first

Vacuum leak from a cracked or disconnected intake hose, intake manifold gasket, or vacuum line
Dirty or failing mass air flow (MAF) sensor reporting less air than actually enters the engine
Weak fuel pump or restricted fuel filter reducing fuel delivery
An exhaust leak ahead of the upstream oxygen sensor that lets outside air reach it and skew the reading
Faulty oxygen sensor (upstream) sending a false lean signal
Dirty or partly clogged fuel injectors on bank 1 that deliver less fuel than commanded
PCV valve or hose leak allowing unmetered air into the intake

How to diagnose and fix it

Check for vacuum leaks by visually inspecting intake hoses, vacuum lines, and the PCV system, and use a smoke machine if available
Clean the mass air flow sensor with the correct cleaner and verify its readings with a scan tool
Test fuel pressure and volume at the fuel rail to rule out a weak pump or clogged filter
Inspect the intake manifold gasket and throttle body gasket for leaks using a smoke test or carburetor cleaner while watching fuel trims
Check the upstream oxygen sensor response with a scan tool to confirm it is switching properly and not biased lean
Perform a propane enrichment test to see if the lean condition improves, indicating a vacuum leak
If all else checks out, inspect injectors for balance and flow, and verify the PCV system is functioning correctly

Before you replace the oxygen sensor

A dirty mass air flow sensor is one of the most common causes of P0171, and cleaning it often resolves the code without any parts replacement.
A vacuum leak downstream of the mass air flow sensor but upstream of the oxygen sensor will cause a lean condition that the computer cannot correct, and these leaks are often invisible without a smoke test.
P0171 can be set by a failing fuel pump that still provides enough pressure at idle but cannot keep up under load, so a fuel pressure test under load is more revealing than a static test.
Exhaust leaks before the upstream oxygen sensor can draw in outside air and cause a false lean reading, so inspect the exhaust manifold and downpipe for cracks or loose fittings.
The code does not always mean the engine is actually running lean; a biased oxygen sensor or a miscalibrated mass air flow sensor can report a lean condition when the mixture is actually correct.

Can I keep driving with P0171?

You can usually drive short distances to a repair shop, but avoid heavy acceleration, towing, or long highway trips because a persistent lean condition can cause overheating and damage to the catalytic converter and spark plugs. If the engine is misfiring or running poorly, stop driving and have it towed.

Tools that help

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