Oxygen Sensor: Description and Operation

The fuel control heated oxygen sensor (HO2S 1) is mounted in the exhaust manifold where the sensor can monitor the oxygen content of the exhaust gas stream. The oxygen present in the exhaust gas reacts with the sensor to produce a voltage output. This voltage should constantly fluctuate from approximately 100 mV at a high oxygen content to 900 mV at a low oxygen content. The heated oxygen sensor voltage can be monitored with a scan tool. By monitoring the voltage output of the oxygen sensor, the PCM calculates what fuel mixture command to send to the injectors.

An open HO2S 1 circuit should set a DTC P0134, and the scan tool will display a constant voltage between 400-500 mV. A constant voltage of less than 175 mV in the sensor circuit should set a DTC P0131, while a constant voltage of more than 975 mV in the circuit should set a DTC P0132. A fault in the HO2S 1 heater circuit should cause a DTC P0135 to set. The PCM can also detect HO2S response problems. If the response time of an HO2S is determined to be too slow, the PCM will store a DTC that indicates a degraded HO2S performance.

To control the emissions of hydrocarbons (HC), Carbon Monoxide (CO), and oxides of Nitrogen (NOx), a 3-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction that oxidizes the HG and CO present in the exhaust gas, converting the HG and CO into harmless water vapor and carbon dioxide. The catalyst also reduces the NOx, converting the NOx to nitrogen. The PCM has the ability to monitor this process using the HO2S 1 and the HO2S 2.

The HO2S 1 produces an output signal that is indicative of the amount of oxygen present in the exhaust gas entering the 3-way catalytic converter. The HO2S 2 produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert the exhaust gases efficiently. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than that produced by the HO2S 2 sensor. The catalyst monitor sensors operate the same as the fuel control sensors.

Although the main function of the HO2S 2 is catalyst monitoring, the sensor also plays a limited role in the fuel control. If the sensor output indicates a voltage of more than or less than the 450 millivolt bias voltage for an extended period of time, the PCM will make a slight adjustment to the fuel trim to ensure that the fuel delivery is correct for the catalyst monitoring.

A problem with the HO2S 2 signal circuit should set a DTC P0137, P0138, or P0140, depending on the specific condition. A fault in the heated oxygen sensor heater element or the heater element's ignition feed or ground will result in slower oxygen sensor response. This may cause erroneous catalyst monitor diagnostic results. A fault in the HO2S 2 heater circuit should cause a DTC P0141 to set.

The heated oxygen sensors (HO2S) require an air reference for proper operation. The air reference is supplied through the HO2S wiring. Inspect the HO2S wires and connections for breaks or contamination. Do not use solder to repair HO2S wiring. Solder will obstruct the air path. Refer to Heated Oxygen Sensor (HO2S) Wiring Repairs in Diagrams for proper repair procedures.