Circuit Description

Heated Oxygen Sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the vehicle is first started the Powertrain Control Module (PCM) operates in an Open Loop mode, ignoring the HO2S signal voltage when calculating the air/fuel ratio. The PCM supplies the HO2S with a reference, or bias, voltage of about 450 millivolts. The HO2S generates a voltage within a range of 0-1000 millivolts that fluctuates at greater than and less than bias voltage once in Closed Loop. A high HO2S voltage output indicates a rich fuel mixture. A low HO2S voltage output indicates a lean mixture. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature, and then provide an accurate voltage signal. The PCM controls the HO2S 1 heater low control circuit with a low side driver. The HO2S 1 heater diagnostic monitors the current draw through the HO2S 1 low side driver when the engine is running. This DTC will set if the current level exceeds a calibrated amount.

Each HO2S 1 has the following circuits: High signal, low reference, heater ignition voltage, heater low control, and low reference loop circuit.