Heated Oxygen Sensors

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-to-fuel ratio. The PCM supplies the HO2S with a reference, or bias, voltage of about 450 mV. The HO2S generates a voltage within a range of 0-1000 mV that fluctuates above and below 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 to provide an accurate voltage signal. If the PCM detects an active HO2S 1 voltage of a lower/higher amount for an excessive amount of time, average response time is too slow, or remains at or near the bias voltage amount, a related Diagnostic Trouble Code (DTC) will set.

The HO2S heater operation is controlled by the PCM. A high side driver supplies ignition power to the bank 1 HO2S 1, and is equipped with a current monitor. Two low side drivers provide the heater ground path. The bank 2 HO2S 1 is supplied power from the external ignition 1 voltage circuit, and uses the same low side drivers. When the ignition switch is turned to RUN position, the PCM performs an HO2S heater circuit check using the high side driver current monitor circuit. When the engine is started, all three drivers are switched ON. One of the low drivers is for heater warm-up only, and is used until the proper heater temperature is reached. When the warm-up low driver is switched OFF, the heater ground path is controlled through the remaining low side driver. The PCM now records the total heater circuit current, if the heater current remains stable the PCM momentarily turns OFF the high side driver. The PCM now records the bank 2 HO2S 1 current. This value is subtracted from the previously recorded total and is used to determine the condition of the heater circuit. If the PCM detects a HO2S sensor heater current outside of the calibrated range for an excessive amount of time, a related DTC will set.