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P0171 code: fuel mixture too lean (bank 1)

Fault code P0171 means the air-fuel mixture on bank 1 is too lean: the engine is getting too much air or too little fuel. The ECM has already tried to correct the mixture by adding fuel, but has reached its limit. The most common cause is air getting into the engine without passing the meter, for example through a cracked hose.

Check P0171

When the code is set

The ECM uses the oxygen sensor to monitor how much oxygen is in the exhaust and adjusts the amount of fuel as it goes. These adjustments are called fuel trim. When it has to add fuel beyond the permitted limit for a long time, it concludes that the mixture is too lean and stores P0171.

Can I keep driving?

Have it checked soon

A short drive to a garage is normally fine, but do not drive the car for long with this code. A lean mixture can damage the engine over time, especially under heavier load. If the engine stalls or jerks, or the check engine light flashes, stop when it is safe.

Symptoms

  • The check engine light is on.
  • Rough idle or stalling when coming to a stop.
  • Less power and poorer response when accelerating.
  • Hard starting.
  • A hissing sound from the engine bay if air is leaking in through a crack.

Common causes

  • An air leak into the intake: a cracked or loose hose, or an intake manifold gasket.
  • A dirty or faulty mass air flow (MAF) sensor.
  • A faulty crankcase ventilation (PCV) valve or its hose.
  • An EVAP purge valve that sticks open.
  • A weak fuel pump or a clogged fuel filter.
  • Clogged injectors.
  • A faulty oxygen sensor (less common; a mechanic tests it before replacing it).

How to find the cause

A mechanic usually works like this:

  1. Read all codes and freeze frame data, and check whether a misfire code is also stored.
  2. Compare fuel trim at idle and at higher revs: high at idle points to an air leak, high at higher revs points to the mass air flow sensor or the fuel supply.
  3. Use a smoke test to find where air is leaking into the intake.
  4. Check the mass air flow sensor and measure the fuel pressure.

Notes by make

BMW
On many BMWs, a common cause of P0171 is a cracked or split rubber hose between the mass air flow sensor and the throttle body.

Other ways the code is written

Scan tools and workshop printouts sometimes show the code with extra characters:

  • P017100

Related codes

Frequently asked questions

Evoca fault code diagnostics is an informational aid and does not replace a professional inspection of your vehicle. Only an inspection of the vehicle at a workshop can confirm the cause of a fault.

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