P0171 — System too lean (bank 1)

What this code generally means, its most common causes, and how serious it is.

TYPE: GENERICSYSTEM: AIR-FUEL MIXTURE

What does the code P0171 mean?

The control module detects more oxygen than expected in bank 1's exhaust: the air-fuel mixture is too lean (not enough fuel for the air coming in). The system continuously adjusts the mixture in the background ("fuel trim"); this code appears once that adjustment hits its limit.

Common possible causes

A vacuum leak — a cracked hose, intake gasket, or PCV valve — the most common cause.
A dirty or failing MAF sensor — underestimates incoming air, often after a poorly maintained oiled air filter.
A weak fuel pump or low fuel pressure — not enough fuel delivered at the right time.
Partially clogged injector(s) — uneven fuel delivery.
An aging oxygen sensor — a skewed reading that throws off the mixture correction.
An exhaust leak upstream of the sensor — lets outside air in before the measurement point.

General severity

Moderate — worth checking soon

A car with this code is usually still drivable, but may idle roughly, feel underpowered, or use more fuel. A mixture that stays very lean for a long time can speed up catalytic converter wear — worth checking soon rather than ignoring.

This is still a general severity for this code, not a diagnostic for your car. Only a real read of your codes, with your mileage and vehicle history, can tell what's really going on.

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Meca doesn't replace an in-person check by a professional. For any serious sign (a flashing light, a major loss of power, a fuel smell), see a mechanic promptly.

Frequently asked questions

The code comes and goes — is that normal?

Yes, that's common with an intermittent vacuum leak (engine movement slightly opens and closes the crack) — the code stays relevant even when it isn't always active.

Can you hear a vacuum leak?

Sometimes: a hiss at idle with a warm engine. But many small leaks are silent and need a real test (smoke or a sprayed brake cleaner test) to pin down.

What's the difference between bank 1 and bank 2?

On an engine with two cylinder rows (V6, V8, some flat-4s), each row has its own bank and sensors. A 4-cylinder inline engine usually only has a bank 1.