Circuit Description

Heated oxygen sensors (HO2S) are used for fuel control and catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the vehicle is started, the control module operates in an Open Loop mode, ignoring the HO2S signal voltage while calculating the air-to-fuel ratio. The control module supplies the HO2S with a reference, or bias, voltage of approximately 450 mV. While the engine runs, the HO2S heats up and begins to generate a voltage within a range of 0-1,100 mV. This voltage will fluctuate above and below the bias voltage. Once sufficient HO2S voltage fluctuation is observed by the control module, Closed Loop is entered. The control module uses the HO2S voltage to determine the air-to-fuel ratio. An HO2S voltage that increases above bias voltage toward 1,100 mV indicates a rich fuel mixture. An HO2S voltage that decreases below bias voltage toward 0 mV indicates a lean fuel mixture.

The heating elements within each HO2S heat the sensor. This allows the system to enter Closed Loop earlier and the control module to calculate air-to-fuel ratio quicker.

The HO2S 2 is used for catalyst monitoring. This diagnostic runs once per ignition cycle. This diagnostic has two phases for running the DTC, a passive test and an intrusive test. If the HO2S 2 voltage transitions below 290 mV and above 685 mV during the passive test, the DTC will pass for this ignition cycle. If the DTC does not pass during the passive test and too much time has elapsed since startup, the intrusive test will begin. The control module will adjust the air-to-fuel ratio during the intrusive test. As the air-to-fuel ratio is adjusted to rich and/or lean, the control module waits for a predicted response from the HO2S 2. If the HO2S 2 voltage transitions below 290 mV and/or above 685 mV, the diagnostic will pass for this ignition cycle. If the control module does not receive the expected response from the HO2S 2, DTC P0136 will set.