Heated Oxygen Sensors

Heated Oxygen Sensors (HO2S) are used for fuel control and post catalyst monitoring. See Fig 1 . 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 that the HO2S voltage did not switch enough times during a calibrated time period, voltage average response time is too slow, loop status is open too long, voltage that stays above or below a specified value, voltage remains at or near the bias voltage amount, heater low control circuit current exceeds a calibrated amount, heater low control circuit current level is not within the calibrated range, heater takes too long to heat or that the calculated transition time ratio is incorrect, a DTC will set.

Each HO2S has the following circuits:

• HO2S high signal.
• HO2S low reference.
• HO2S heater ignition voltage.
• HO2S heater low control.
• Low reference loop.

Fig 1: Locating Heated Oxygen Sensors

Courtesy of GENERAL MOTORS CORP.