The Heated Oxygen Sensor (HO2S) is a device that 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 ECM.
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 ECM 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 ECM 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 ECM is running in what as known as open loop operation. Open loop means that the ECM has not yet begun to correct the air-to-fuel ratio by reading the oxygen sensor. After the engine comes to operating temperature, the ECM 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
- Raise and safely support the vehicle.
- Unplug the sensor connector. Remove the clip from the harness.
- Remove the sensor from the exhaust pipe or catalytic converter.
Installation is the reverse of removal. If reusing the old sensor, coat the threads with GM antiseize compound 12377953 or equivalent. New sensors are already coated; additional compound is not needed. Tighten the sensor to 30 ft. lbs. (40 Nm).
- With the ignition OFF , disconnect the harness connector at the appropriate HO2S.
- With the ignition ON , verify that a test lamp illuminates between the appropriate HO2S heater voltage supply circuit 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.
- With the ignition ON , verify that a test lamp does not illuminate between the appropriate HO2S heater voltage supply control circuit and the appropriate HO2S heater low control circuit. If the lamp illuminates, test the HO2S heater low control circuit for a short to ground.
- 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.
- With the ignition OFF , install a 30A fused jumper wire between the appropriate HO2S heater voltage supply circuit and the appropriate HO2S heater low control circuit. With the engine running, verify the appropriate scan tool HO2S Heater parameter is less than 0.1 amp. 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 ECM and all circuits test normal, replace the appropriate HO2S.
- Allow the engine to reach operating temperature.
With the engine running, observe the affected HO2S parameter with a scan tool.:
- The pre-catalyst oxygen sensors value should vary from below 200 mV to above 800 mV and respond to fueling changes.
- The post-catalyst oxygen sensors value should change more than 200 mV when the throttle is quickly cycled 3 times from closed to wide open and back to closed after running the engine at 1,500 RPM for 30 seconds.
- If the sensor did not perform as indicated, replace the sensor.