Circuit/System Testing

Ignition OFF, disconnect the harness connector of the appropriate HO2S.
Ignition ON, verify that the HO2S parameter is between 350-500 mV.
- If greater than 500 mV, test the signal circuit of the HO2S for a short to voltage. If the circuit/connections test normal, replace the engine control module (ECM).

IMPORTANT: The test lamp is used to load the circuit and may not illuminate.

Connect a test lamp between the low reference circuit of the HO2S and the positive terminal of the battery.
Ignition ON, test for less than 0.1 volt between the low reference circuit of the HO2S sensor and the negative battery terminal.
- If greater than 0.1 volt, test the low reference circuit of the HO2S for a short to voltage. If the circuit/connections test normal, replace the ECM.
Verify that the following conditions do not exist:
- Water intrusion in the HO2S harness connector
- High fuel system pressure-Refer to Fuel System Diagnosis .
- Rich fuel injectors or fuel injectors that are leaking-Refer to Fuel Injector Balance Test with Special Tool .
- Fuel saturation of the evaporative emission (EVAP) canister
- Exhaust system restrictions
- Leaking fuel pressure regulator-Inspect for evidence of fuel in the vacuum line. Refer to Fuel System Diagnosis .
- If you find any of the above conditions, repair as necessary.
If all circuits/connections test normal, replace the applicable HO2S.