P0172

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

NOTE: If some of the DTCs listed below are stored at the same time as DTC P0171 and/or P0172, troubleshoot those DTCs first, then recheck for P0171 and/or P0172.
P0107, P0108, P1128, P1129: Manifold Absolute
Pressure (MAP) sensor
P0135: Primary Heated Oxygen Sensor (primary HO2S) (Sensor 1) heater
P0137, P0138: Secondary HO2S (Sensor 2)
P0141: Secondary HO2S (Sensor 2) heater
P0401, P1491, P1498: Exhaust Gas Recirculation (EGR) system
P1259: VTEC system

1. Check the fuel pressure.

Is the fuel pressure OK?

YES - Go to step 2.

NO - Check these items:
- If the pressure is too high, check the fuel pressure regulator, and the fuel return pipe.
- If the pressure is too low, check the fuel pump, the fuel feed pipe, and the fuel filter, and the fuel pressure regulator.

2. Start the engine. Hold the engine at 3,000 rpm with no load (in Park or neutral) until the radiator fan comes on.
3. Check the primary HO2S (Sensor 1) output with the scan tool.

Does it stay less than 0.3 V or more than 0.6 V?

YES - Replace the primary HO2S (Sensor 1).

NO - Go to step 4.

4. Turn the ignition switch OFF.
5. With a vacuum pump, apply vacuum to the Evaporative Emission (EVAP) canister purge valve from the intake manifold side.

Does it hold vacuum?

YES - Check the valve clearances and adjust if necessary. If the valve clearances are OK, replace the injectors.

NO - Replace the EVAP canister purge valve.