Circuit/System Verification

If any HO2S heater DTCs are set, perform those diagnostics first.
Allow the engine to reach operating temperature.
Engine running, observe the affected HO2S parameter with a scan tool.
- The pre-catalyst oxygen sensors value should vary from below 200 mV and respond to fueling changes.
- The post-catalyst oxygen sensors value should change more than 200 mV when the throttle is quickly cycled 3 times from closed to wide open and back to closed after running the engine at 1,500 RPM for 30 seconds.
The affected HO2S may be damaged due to contamination. Prior to replace the affected HO2S, inspect for the following sources of contamination:
- A silicon contaminated HO2S
- Fuel contamination-Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
- Engine oil consumption-Refer to Oil Consumption Diagnosis .
- Engine coolant consumption-Refer to Loss of Coolant .