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FuelSeverity Medium: diagnose soon

P0171System Too Lean (Bank 1)

What it means

The ECU is adding much more fuel than normal on bank 1 and has reached its limit: the engine is getting too much air or too little fuel.

Symptoms

  • Rough or hunting idle
  • Hesitation when accelerating
  • Sometimes misfires

Common causes

  • Vacuum or unmetered air leak after the MAF (gaskets, hoses, PCV valve)
  • MAF sensor under-reading (dirty)
  • Low fuel pressure: weak pump, clogged filter
  • Restricted injectors
  • Exhaust leak before the upstream oxygen sensor

What to check first

  1. Compare long-term fuel trims at idle and at 2,500 rpm: high at idle only points to a vacuum leak, high everywhere to MAF or fuel supply
  2. Smoke-test the intake
  3. Check fuel pressure under load
Can I drive with P0171? Usually yes for a short distance, but performance or economy may suffer and the fault can get worse. Diagnose it soon.
Practise it: Vacuum leak in the engine simulator shows this fault in live data, tests and on the scope.Open in the simulator

Generic OBD-II meaning. Manufacturer-specific meanings and checks can differ; always check the service information for the car.

How to test the parts behind P0171

Mass air flow sensor (MAF)Wiring, typical values, multimeter and scope testsOxygen (lambda) sensorWiring, typical values, multimeter and scope testsPetrol fuel injector (port injection)Wiring, typical values, multimeter and scope tests

Known engine problems that set P0171

1.8/2.0 TFSI EA888: Crankcase breather (PCV) valve failureAudi, Volkswagen, Skoda, Seat · typically 60,000–160,000 kmVolvo 5-cyl petrol: Clogged crankcase ventilation (oil trap)Volvo, Ford · typically 100,000–200,000 km

Cases with P0171 (1)

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Whistle at idle, slightly rough idle and a lean codeTraining case · 2012 Volkswagen Passat B7 · 1.8 TSI (CDAB) · fixed: Split diaphragm in the crankcase breather (PCV) valve, letting in unmetered air

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P0171: questions and answers

What does P0171 mean?

P0171: System Too Lean (Bank 1). The ECU is adding much more fuel than normal on bank 1 and has reached its limit: the engine is getting too much air or too little fuel.

What are the symptoms of P0171?

Rough or hunting idle. Hesitation when accelerating. Sometimes misfires.

What causes P0171?

Vacuum or unmetered air leak after the MAF (gaskets, hoses, PCV valve). MAF sensor under-reading (dirty). Low fuel pressure: weak pump, clogged filter. Restricted injectors. Exhaust leak before the upstream oxygen sensor.

How do you diagnose P0171?

1. Compare long-term fuel trims at idle and at 2,500 rpm: high at idle only points to a vacuum leak, high everywhere to MAF or fuel supply 2. Smoke-test the intake 3. Check fuel pressure under load

Can I drive with P0171?

Usually yes for a short distance, but performance or economy may suffer and the fault can get worse. Diagnose it soon.