DTC P0171 Or DTC P0172

Code Definition

• DTC P0171: Fuel System Too Lean
• DTC P0172: Fuel System Too Rich

1. 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 line. Repair as necessary. If fuel pressure is too low, check fuel pump, fuel feed line, fuel filter and fuel pressure regulator. Repair as necessary.
2. Start engine and run at 3000 RPM, with no load (in park or neutral) until radiator fan comes on. Using scan tool, check Primary Heated Oxygen Sensor (HO2S-1) output data. If voltage stays at less than 0.3 volt or more than 0.6 volt, replace HO2S-1. If voltage does not stay at less than 0.3 volt or more than 0.6 volt, go to next step.
3. Turn ignition switch off. Using a vacuum pump, apply vacuum to intake manifold side of EVAP canister purge valve. If EVAP canister purge valve holds vacuum, check valve clearances. Adjust valves as required. If valve clearances are okay, replace fuel injectors. If EVAP canister purge valve does not hold vacuum, replace EVAP canister purge valve.