Fuelling · Fault code

P0171

System Too Lean (Bank 1)

The fuelling control system has hit its lean-correction limit on bank 1 — it has been adding fuel to compensate for something and has run out of adjustment. It is a mixture-control threshold, not a reading from any one sensor.

Severity EXPERT Moderate — diagnose before it gets worse
Driveability EXPERT Drive with caution — the ECU is already protecting itself
Tuning EXPERT Do not tune a car storing this — fix it first

Check first — in this order

The order is the product. A list of "possible causes" is what every parts site publishes, and it is why people replace parts blindly.

  1. Compare short and long fuel trims at idle against trims at 2500 rpm. A large idle-only error points somewhere different from an error that persists under load. EXPERT
  2. On a modern turbo petrol engine, check crankcase ventilation first. A torn PCV diaphragm or a failed valve-cover breather is the most common single cause on N54, N55 and EA888, and it is invisible to a quick visual check. EXPERT
  3. Smoke- or pressure-test the intake for unmetered air. EXPERT
  4. Check the airflow signal for plausibility before touching fuelling hardware. EXPERT
  5. Verify fuel delivery under load — rail pressure holding at full load, not just at idle. EXPERT

What this code does not license you to replace

  • That the oxygen sensor is at fault. It is usually reporting the situation accurately.
  • That fuel injectors are the cause before unmetered air and crankcase ventilation have been excluded.

Companion codes

Codes whose presence alongside P0171 changes the path.

P0299P0101P0106

Engines we cover that log this

EA888 Gen 3 Wastegated (petrol)BMW N54 Wastegated (petrol)PSA/BMW THP 1.6 Wastegated (petrol)
No workshop sign-off yet. The diagnostic order on this page has had a technical review of the knowledge model, performed by a model — not a manufacturer certification and not a mechanical inspection of any vehicle. No tuner has yet put their name to this ordering. Treat it as a reasoned starting point, confirm with measurement, and get a professional to sign off before you condemn a part.

Built by DMS Tuned

Stuck on P0171? Send us the log.

DMS Tuned is a real ECU-tuning shop. If you have a boost/requested-vs-actual log, run it through the analyser first — then message us and we will read it with you before you spend money on parts.

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