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P0171

System too lean (Bank 1)

P0171 indicates that the engine control system has detected a lean air-fuel mixture on Bank 1.

What this code means

P0171 indicates that the engine control system has detected a lean air-fuel mixture on Bank 1.

Possible causes

  • Vacuum or intake leak
  • Dirty or faulty mass-airflow sensor
  • Fuel-delivery restriction or low pressure
  • Injector issue
  • Oxygen-sensor issue
  • Exhaust leak ahead of the oxygen sensor

Common symptoms

  • Check-engine light
  • Rough idle or stalling
  • Reduced power or poor acceleration
  • Hard starting
  • Possible knocking or pinging

Checks to make

  1. Check for related stored codes
  2. Inspect intake and vacuum hoses for leaks
  3. Review short-term and long-term fuel trims with the engine warm
  4. Check MAF readings and fuel-delivery data as appropriate

Safety

  • If the engine is stalling, running very poorly or showing signs of overheating or detonation, avoid continued driving until the fault is assessed.

Model notes

Bank 1 is the side of the engine containing cylinder 1; many inline engines have only one bank.