P0171: System Too Lean (Bank 1)

P0171 means the air-fuel mixture on Bank 1 (the cylinder bank with cylinder 1) is too lean: too much air or too little fuel. The ECU keeps adding fuel to hit the 14.7:1 target ratio, but it has run out of room to compensate, so long-term fuel trim climbs past about +15% and the light comes on. The single most common cause is unmetered air entering after the airflow sensor, usually a vacuum or intake leak. Most fixes land between 100 and 350 EUR.

P0171 is a fuel system-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0171 and illuminates the check engine light.

If your vehicle has triggered P0171, you may notice one or more of these symptoms:

• Check engine light illuminated (often steady, flashing only if a lean misfire develops)
• Rough, high, or surging idle
• Hesitation or stumble during acceleration
• Reduced fuel economy, because the ECU dumps in extra fuel to chase the correct ratio
• Engine may stall at idle or feel like it almost stalls when stopping
• Hissing or whistling sound from a vacuum leak under the hood
• Occasional lean misfire, which can set P0300 or a cylinder-specific misfire code

Here are the most common causes of P0171, ranked by how likely they are to be the culprit:

Generally yes, for short-term driving. Short-term driving is generally safe, but a lean condition can cause engine overheating and damage if left unaddressed. Avoid heavy acceleration and fix within a week.

Follow these steps to pinpoint the root cause of P0171:

1. Read short-term and long-term fuel trims with a scan tool. Long-term fuel trim above roughly +15% at idle confirms the lean condition, and if it improves at higher RPM the leak is unmetered air at idle
2. Smoke-test the intake, or spray brake cleaner or carb cleaner around the intake manifold gasket, vacuum hoses, and PCV connections. A momentary RPM change pinpoints the leak
3. Inspect and clean the MAF sensor with dedicated MAF cleaner, never touching the sensor element, then recheck fuel trims
4. Check fuel pressure at the rail with a gauge and compare to the manufacturer specification, watching for pressure drop under load that points to a weak pump or clogged filter
5. Inspect the PCV valve, oil separator, and their hoses for cracks, splits, or disconnection
6. Check for an exhaust leak just before the upstream O2 sensor, which can fool the sensor into a false lean reading
7. Compare Bank 1 and Bank 2 trims on V engines. If both banks read lean, suspect a shared cause such as the MAF, fuel supply, or a leak before the throttle body

Repair costs for P0171 vary depending on the root cause and your vehicle. Here are typical estimates:

P0171 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:

These codes are often seen alongside P0171 or indicate related issues: