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DTC P0171 & DTC P0172: Fuel Supply System

NOTE: If DTCs P0107, P0108, P0135, P0137, P0138, P0141, P1128, P1129, P1456, or P1457 are indicated, trouble shoot DTC P171 and/or DTC P172 last.
NOTE: DTC P0171 and/or P0172 is set and MIL illuminated when malfunction has been detected in long term fuel trim, during 2 consecutive trips.
  1. If DTC P0171 (system to lean) is set, check for:
    • Insufficient fuel pump flow or pressure.
    • Clogged or leaking fuel feed line.
    • Fuel pressure regulator stuck open.
    • Clogged fuel filter.
    • Clogged or blocked fuel injector or air leakage.
    • Fuel octane level less than specification.
    • A deteriorated HO2S-1.
    • Valve adjustment out of specification. See ON-VEHICLE ADJUSTMENTS article.
    • An exhaust system leak.

    Repair as necessary. If no problems are found, go to step  3 .

  2. If DTC P0172 (system to rich) is set, check for:
    • Fuel pressure regulator clogged or stuck closed.
    • Clogged fuel return line.
    • A leaking fuel injector.
    • Fuel octane level less than specification.
    • A deteriorated HO2S-1.
    • A leaking or stuck open EVAP purge control solenoid valve.
    • Valve adjustment out of specification. See ON-VEHICLE ADJUSTMENTS article.

    Repair as necessary. If no problems are found, go to next step.

  3. Check fuel pressure. See FUEL SYSTEM in BASIC DIAGNOSTIC PROCEDURES article. If fuel pressure is okay, go to next step. If fuel pressure is too high, check fuel pressure regulator and fuel return pipe. Repair as necessary. If fuel pressure is too low, check fuel pump, fuel feed pipe, fuel filter and fuel pressure regulator. Repair as necessary.
  4. Start engine and run at 3000 RPM with no loads and transmission in Park or Neutral until radiator fan comes on. Using scan tool, check HO2S-1 output voltage. If voltage does not stay at less than 0.3 volt or more than 0.6 volt, go to next step. If voltage stays at less than 0.3 volt or more than 0.6 volt, replace HO2S-1.
  5. Using a vacuum pump, apply vacuum to EVAP purge control solenoid valve from intake manifold side. If vacuum does not hold, replace EVAP purge control solenoid valve. If vacuum holds, ensure valve clearances are correct. If valve clearances are correct, replace fuel injectors.
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