For 1990-2009 cars

### Circuit Description

The rear heated oxygen (HO2S 2) sensor consists of a zirconia electrolyte between 2 platinum plates. When the reaches approximately 318°C (600°F) it becomes an electrical source that responds to the oxygen content in the exhaust. The powertrain control module (PCM) produces a bias voltage of approximately 450 millivolts on the oxygen sensor circuit. When the sensor is clod its internal resistance is extremely high, therefore, the PCM recognizes the bias as an open circuit. As the sensor warms up, the internal resistance decreases. When the sensor reaches approximately 318°C (600°F), it starts producing a voltage based on the oxygen content in the exhaust stream. Refer to DTC P0141 fior heater circuit operation.

### Conditions for Setting the DTC

DTC P0141 will set if the HO2S-2 voltage does not change by more than 150 millivolts after 100-190 seconds, exact time depends on start-up coolant temperature and airflow after start-up, when:

• | The engine coolant temperature and intake air temperature are within 10°C (50°F) of each other.

• | The average air flow is less than 17 grams/second.

• | The test is run once per cycle.

### Test Description

1. Since the heater is powered by the ignition, the voltage must be viewed immediately.

2. The PCM supplies a bias voltage of 350-550 millivolts to the sensor. If the sensor is too hot due to converter temperature or ignition ON too long this voltage could be above 550 millivolts or below 350 millivolts.

3. If it has been verified the exhaust is cool at the sensor, and the voltage is above or below the specified value it could be due to leakage current between wires or possible leakage inside the sensor 4-way connector. The wire routing and connector should be carefully inspected.

4. With the engine not running, there should mainly be oxygen inside the exhaust as indicated by the voltage going down. However, depending on the amount of contaminates on the sensor this voltage could go up due to the heater causing this residue to be burned off using up any oxygen around the sensor.

Important: Do not apply grease or any material in the HO2S-2 connector or alter the wiring in any way. The wires carry reference air to the sensor used to determine the difference in oxygen from the exhaust gas to the PCM.