Heated Oxygen Sensors

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 to 900 mV. The heated oxygen sensor voltage can be monitored with a scan tool. By monitoring the voltage output of the oxygen sensor, the PCM calculates the fuel mixture command to give to the injectors. A lean mixture and a low HO2S voltage prompt a rich command. A rich mixture and a high HO2S voltage prompt a lean command.

If the HO2S 1 circuit is open, DTC P0134 should set and the scan tool display a constant voltage between 400 - 500 mV. A constant voltage below 300 mV in the sensor circuit should set DTC P0131, while a constant voltage above 800 mV in the circuit should set DTC P0132. A fault in the HO2S 1 heater circuit should cause 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 degraded HO2S performance.

To control 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 which converts the HC and the CO present in the exhaust gas into harmless water vapor and carbon dioxide. The catalyst also converts the NOx to nitrogen. The PCM has the ability to monitor this process using the HO2S 1 and the HO2S 2 heated oxygen sensors.

The HO2S 1 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the 3-way catalytic converter. The HO2S 2 sensor 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 HO2S 1 signal is 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 sensors is catalyst monitoring, the sensors also play a limited role in fuel control. If the sensor output indicates a voltage more than or less than the 450 millivolt bias voltage for an extended period of time, the PCM makes a slight adjustment to the fuel trim in order to ensure that the fuel delivery is correct for catalyst monitoring.

A problem with the HO2S 2 signal circuit should set DTC P0137, DTC P0138 or DTC P0140, depending upon the specific condition. A fault in an HO2S heater element, 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 DTC P0141 to set.