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 in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The Powertrain Control Module (PCM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first started the PCM operates in open loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperatures and Closed Loop is achieved, the HO2S generates a voltage within a range of 0-1000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream. Low HO2S voltage indicates a lean exhaust stream.

The HO2S must reach operating temperature to provide an accurate voltage signal. A heating element inside the HO2S minimizes the time required for the sensor to reach operating temperature. Voltage is provided to the heater by the ignition 1 voltage circuit through a fuse. With the engine running, ground is provided to the heater by the HO2S heater low control circuit, through a low side driver within the PCM. The PCM monitors the current flow through the heater for diagnosis.

If the oxygen sensor pigtail wiring, connector, or terminal are damaged, replace the entire oxygen sensor assembly. Do not attempt to repair the wiring, connector, or terminals. For the sensor to function properly, the sensor must have a clean air reference. This clean air reference is obtained by the oxygen sensor wires. Any attempt to repair the wires, the connectors, or the terminals could result in the obstruction of the air reference. Any attempt to repair the wires, the connectors, or the terminals could degrade oxygen sensor performance.

To control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides Of Nitrogen (NOx), the system uses a 3-way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting the HC and the CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting the NOx into nitrogen.

The PCM has the ability to monitor this process using the HO2S mounted after the 3-way catalytic converter. The HO2S produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalysts ability to convert the exhaust gases efficiently. If the catalyst is operating efficiently, the O2S signal will be far more active than that produced by the HO2S.