P0171 Fault Code: What It Means and What To Do

What does P0171 mean?

P0171 means the engine management system has detected that the air/fuel mixture on bank 1 is running lean — there is too much air relative to the amount of fuel being burned. The ECU monitors the mixture using oxygen sensors; when it detects lean conditions persistently and has to add extra fuel (called "positive fuel trim") to compensate, it logs P0171.

Bank 1 refers to the side of the engine containing cylinder 1. On a four-cylinder engine there's only one bank, so P0171 affects the whole engine. On a V6 or V8, bank 2 has its own code (P0174).

Common symptoms include rough or unstable idle, hesitation when accelerating, increased fuel consumption, and occasionally a smell of fuel or a lean pop when the throttle is lifted.

Common causes

• Vacuum leak — cracked intake hose, split vacuum pipe, or a failed intake manifold gasket lets unmetered air into the engine, leaning out the mixture. This is the single most common cause.
• Dirty or faulty MAF sensor — the mass airflow sensor measures how much air is entering the engine. A dirty or failing MAF under-reports airflow, causing the ECU to inject too little fuel.
• Weak fuel pump or low fuel pressure — if the pump can't maintain adequate pressure, injectors can't deliver enough fuel, especially under load.
• Clogged fuel filter — restricts fuel flow, reducing the pressure available at the injectors.
• Faulty upstream O2 sensor — a sensor stuck in a lean state sends incorrect signals, causing the ECU to think the mixture is lean when it may not be.
• Leaking injectors — on some engines, injectors that don't seal properly between pulses cause the fuel rail pressure to drop.

Typical UK repair costs

UK labour rates are typically £80–£120 per hour. Costs vary by vehicle and region.

What to do next

Start with the cheapest possibilities. Check all vacuum hoses visually for cracks, splits, or loose connections — paying particular attention to the large intake hose between the air filter box and throttle body. If you have access to an OBD scanner, check the short-term and long-term fuel trim values: large positive values (above +10%) confirm the ECU is struggling to compensate for the lean condition.

If hoses look fine, try cleaning the MAF sensor with proper MAF cleaner spray before paying for a replacement. If the fault persists after cleaning, a proper diagnosis with a smoke test to find vacuum leaks is the most efficient next step.