Heated Oxygen Sensors

The fuel control Heated Oxygen Sensors (Bank 1 HO2S 1 and Bank 2 HO2S 1) are mounted in the exhaust manifolds where they 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 100mV (high oxygen content - lean mixture) to 900mV (low oxygen content - rich mixture). The heated oxygen sensor voltage can be monitored with a scan tool.

By monitoring the voltage output of the oxygen sensor, the PCM can calculate what fuel mixture command to send the injectors (lean mixture-low HO2S voltage = rich command, rich mixture-high HO2S voltage = lean command).

An open Bank 1 HO2S 1 circuit should set a DTC P0134; an open in Bank 2 HO2S 1 circuit should set a DTC P0154. With an open HO2S signal, the scan tool will display a constant voltage between 400 - 500mV. A constant voltage below 300mV in the Bank 1 HO2S 1 sensor circuit (circuit grounded) should set DTC P0131. A constant voltage below 300mV in the Bank 2 HO2S 1 sensor circuit (circuit grounded) should set DTC P0151. A constant voltage above 800mV in the Bank 1 HO2S 1 circuit should set DTC P0132, while a constant voltage above 800mV in the Bank 2 HO2S 1 circuit should set DTC P0152. A fault in the Bank 1 HO2S 1 heater circuit should cause DTC P0135 to set. A fault in the Bank 2 HO2S 1 heater circuit should cause DTC P0155 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 three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The PCM has the ability to monitor this process using the Bank 1 HO2S 2. The Bank 1 HO2S 1 or Bank 2 HO2S 1 produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The Bank 1 HO2S 2 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the Bank 1 HO2S 1 and the Bank 2 sensor 1 signals will be far more active than that produced by the Bank 1 HO2S 2 sensor.

Although the Bank 1 HO2S 2 sensors main function is catalyst monitoring, they also plays a limited role in fuel control. If the sensor output indicates a voltage either above or below the 450 millivolt bias voltage for an extended period of time, the PCM will make a slight adjustment to fuel trim to ensure that fuel delivery is correct for catalyst monitoring.

A problem with the Bank 1 HO2S 2 signal circuit should set DTC P0137, P0138 or P0140 depending on the specific condition. A fault in the heated oxygen sensor heater element or its ignition feed or ground will result in slower oxygen sensor response. This may cause erroneous catalyst monitor diagnostic results. A fault in the Bank 1 HO2S 2 heater circuit should cause DTC P0141 to set.