Circuit Description

PCM supplies a bias voltage (about 450 millivolts) on the HO2S high and low signal circuits. When ignition is turned on, battery voltage is supplied to HO2S heater. As the heater reaches operating temperature, HO2S voltage responds by changing from bias voltage range to normal operation (less than 300 mV). Depending on exhaust gas content, it is possible for the HO2S voltage to be greater than 450 mV.

PCM will only run heater test only on a cold start, depending on cumulative air flow, and only once an ignition cycle. When engine is started, PCM will monitor HO2S voltage. When HO2S goes greater than or less than the bias range threshold, PCM will determine how much time it took. If PCM detects that it took too much time for HO2S to enter into normal operating range, DTC will set. The time it takes HO2S to reach operating temperature is based on the amount of air that flows into the engine.