The Heated Oxygen Sensor (HO2S) is a device which produces an electrical voltage when exposed to the oxygen present in the exhaust gases. The oxygen sensors are electrically heated internally for faster switching when the engine is started cold. The oxygen sensor produces a voltage within 0 and 1 volt. When there is a large amount of oxygen present (lean mixture), the sensor produces a low voltage (less than 0.4v). When there is a lesser amount present (rich mixture) it produces a higher voltage (0.6-1.0v). The stoichiometric or correct fuel to air ratio will read between 0.4 and 0.6v. By monitoring the oxygen content and converting it to electrical voltage, the sensor acts as a rich-lean switch. The voltage is transmitted to the PCM.
Two sensors per bank are used, one before the catalyst and one after. This is done for a catalyst efficiency monitor that is a part of the diagnostic system of the engine controls. The one before the catalyst measures the exhaust emissions right out of the engine, and sends the signal to the PCM about the state of the mixture as previously talked about. The second sensor reports the difference in the emissions after the exhaust gases have gone through the catalyst. This sensor reports to the PCM the amount of emissions reduction the catalyst is performing.
The oxygen sensor will not work until a predetermined temperature is reached, until this time the PCM is running in what as known as open loop operation. Open loop means that the PCM has not yet begun to correct the air-to-fuel ratio by reading the oxygen sensor. After the engine comes to operating temperature, the PCM will monitor the oxygen sensor and correct the air/fuel ratio from the sensor's readings. This is what is known as closed loop operation.
Removal & Installation
- Remove the heat shield.
- Note the routing of the Heated Oxygen Sensor (HO2S) electrical harness.
- Disconnect the HO2S electrical connector.
- Using a suitable Oxygen sensor socket remove the sensor.
- If reinstalling the old HO2S, coat the threads with anti-seize compound.
- Carefully install the HO2S to the pipe. Tighten the sensor to 22 ft. lbs. (30 Nm) on the sensor located at exhaust manifold, or 30 ft. lbs. (41 Nm) for the rear oxygen sensor.
- Attach the sensor electrical connections.
- Turn the ignition OFF. Disconnect the harness connector at the appropriate HO2S.
- Turn the ignition ON, verify that a test lamp illuminates between the appropriate HO2S heater voltage supply circuit terminal D and ground. If the test lamp does not illuminate, test the HO2S heater voltage supply circuit for a short to ground or an open/high resistance. If the circuit tests normal and the HO2S heater voltage supply circuit fuse is open, test all components connected to the fuse and replace as necessary.
- Turn the ignition ON, verify that a test lamp does not illuminate between the appropriate HO2S heater low control circuit terminal C and the HO2S heater voltage supply circuit terminal D. If the lamp illuminates, test the HO2S heater low control circuit for a short to ground. If the circuit tests normal, replace the PCM.
- With the engine running, leave the test lamp connected from the previous step. The lamp should flash or be ON steady. If the test lamp is not on steady or flashing, test the HO2S heater low control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the PCM.
Turn the ignition off, install a 30A fused jumper wire between the appropriate HO2S heater low control circuit terminal C and the HO2S heater voltage supply circuit terminal D. Engine running, verify the appropriate scan tool Heater parameter is less than 0.1A. If more than the specified range, test the HO2S heater voltage supply and HO2S heater low control circuits for more than 1 ohm of resistance. If the circuits test normal, replace the PCM. If the PCM and all circuits test normal, replace the appropriate HO2S.