P0171 System Too Lean (Bank 1)

Powertrain Severity: Moderate Generic OBD-II

The engine computer is adding a lot of extra fuel on bank 1 because it sees more air than expected or too little fuel.

Common causes

  • Vacuum leak (hoses, intake gasket, brake booster)
  • Dirty or faulty MAF sensor
  • Weak fuel pump or clogged filter
  • Leaking PCV valve or hose
  • Exhaust leak before the upstream O2 sensor

Symptoms

  • Check engine light
  • Hesitation
  • Rough idle
  • Possible misfires

What to check

  1. Check long-term fuel trim at idle and at 2500 rpm
  2. Smoke-test the intake for leaks
  3. Clean or test the MAF
  4. Measure fuel pressure

Manufacturers may add their own detail to generic codes. If your vehicle shows a different description for P0171, follow the service information for that make and model.

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