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Air, fuel and mixture

P0171Poor mix — bank 1

Meaning

The computer is adding more fuel than normal to maintain the correct mixture and has reached the limit of the correction. There is air entering the engine that is not metered, or the fuel is insufficient.

Urgency

Average. The car usually drives normally, but the lean mixture overheats the engine and catalytic converter and causes misfires.

It affects

Gasoline engines most often. Intake, vacuum hoses, flow meter, fuel system, lambda probe.

What does P0171 mean

The lambda sensor in front of the catalyst monitors how much oxygen remains in the exhaust gases. If there is too much, the computer assumes the mixture is lean and extends the injection. This fix is called a "fuel fix" (fuel trim). When it exceeds about +25 %, the computer records P0171.

"Bank 1" is the 1 cylinder engine side. For inline engines, there is only bank 1. If the same thing happens on the other side with the V- engine, it also comes out P0174 — then the cause is usually common to the entire engine.

Symptoms

  • "Check Engine" light bulb, often with no other visible problem
  • Rough idle or "floating" revs
  • Stuttering and poor throttle response
  • Hard to start, especially when cold
  • Misfires (P0300) at a more severe lean mixture

| Can it be driven?

Yes, briefly. If the car runs normally, you can get to the repair shop. Don't delay for weeks: a lean mixture burns hotter, damages the valves and catalytic converter, and can hide a more serious problem.

Common reasons

  1. Vacuum leak — cracked or hardened hose, loose clamp, intake manifold gasket. The most common reason.
  2. Crankcase ventilation (PCV) — a ruptured diaphragm or hose.
  3. Dirty or faulty flow meter (MAF) — reports less air than the real one (see P0101).
  4. Air leak after the flow meter — cracked sleeve or leaky air filter.
  5. Low fuel pressure — weak pump, clogged filter, regulator.
  6. Clogged nozzles.
  7. Leak in the exhaust in front of the lambda probe — sucks in air and the probe reads "lean".

How we diagnose it

  1. We read the codes and look at the short and long term fuel correction at idle and revs.
  2. If the mixture is lean only at idle — we look for a vacuum leak (smoke test).
  3. If it is lean under load — we check the flow meter and fuel pressure.
  4. We check the lambda probe and exhaust for leaks.