Heated oxygen sensors are used to minimize the amount of time required for closed loop fuel control operation and to allow accurate catalyst monitoring. The oxygen sensor heater greatly decreases the amount of time required for fuel control sensor HO2S 1 to become active. The oxygen sensor heater is required by catalyst monitor sensor HO2S 2 to maintain a sufficiently high temperature. This allows accurate exhaust oxygen content readings further from the engine.

The PCM will run the heater test only after a cold start (determined by engine coolant and intake air temperature at the time of startup) and only once during an ignition cycle. When the engine is started the PCM will monitor the HO2S voltage. When the HO2S voltage indicates a sufficiently active sensor, the PCM looks at how much time has elapsed since startup. If the PCM determines that too much time was required for the HO2S 1 to become active, a DTC P0135 will set. The time it should take the HO2S to reach operating temperature is based on the engine coolant temperature at start-up and the average Mass Air Flow since startup (higher average airflow or higher startup engine coolant temperature = shorter time to HO2S activity).

HO2S 1 voltage remains within 150 mV of the bias voltage (about 450 mV) for a longer amount of time than it should. The amount of time ranges between 50 and 80 seconds depending on engine coolant temperature at startup and average Mass Air Flow since startup.

Check for the following conditions:

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

The numbers below refer to the step numbers on the diagnostic table:

2. The HO2S should be allowed to cool before performing this test. If the HO2S heater is functioning, the signal voltage will gradually increase or decrease as the sensor element warms. If the heater is not functioning, the HO2S signal will remain near the 450mv bias voltage.

4. This step ensures that the ignition feed circuit to the HO2S is not open or shorted. The test lamp should be connected to a good ground, in case the HO2S low or HO2S heater ground circuit is faulty.

5. This step tests the HO2S heater ground circuit.

6. This step checks for an open or shorted HO2S heater element. The heater element resistance will vary according to HO2S temperature (a hot HO2S heater element will measure a much higher resistance than a HO2S heater element at room temperature). It is important to allow the HO2S to cool before measuring HO2S heater element resistance.