Circuit Description

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

PCM will run heater test only on a cold start and once during the ignition cycle. When engine is started, PCM will determine how much time it takes HO2S voltage to go from above or below the bias range threshold. If PCM detects that it took too much time to enter into normal operating range, DTC will set. The time it takes the HO2S to reach operating temperature is based on the amount of air that flows into the engine.