As a vehicle accrues mileage, the catalytic converter deteriorates. The deterioration results in a less effective catalyst. To monitor catalytic converter deterioration, the fuel injection system uses two heated oxygen sensors: one is upstream of the catalytic converter and one downstream of the converter.
The heated oxygen sensor, or HO2S sensor is usually located near the catalytic converter. It produces a voltage signal of 0.1-1.0 volts based on the amount of oxygen in the exhaust gas. When a low amount of oxygen is present (caused by a rich air/fuel mixture), the sensor produces a high voltage. When a high amount of oxygen is present (caused by a lean air/fuel mixture), the sensor produces a low voltage. Because an accurate voltage signal is only produced if the sensor temperature is above approximately 600F (315C), a fast-acting heating element is built into its body.
The PCM uses the HO2S sensor voltage signal to constantly adjust the amount of fuel injected that keeps the engine at its peak efficiency.
The PCM compares the reading from the sensors to calculate the catalytic converter oxygen storage capacity and storage efficiency. The PCM also uses the upstream heated oxygen sensor input when adjusting the injector pulse width. When the catalytic converter efficiency drops below preset emission criteria, the PCM stores a Diagnostic Trouble Code (DTC) and illuminates the Malfunction Indicator Lamp (MIL).
The automatic shutdown relay supplies battery voltage to both of the heated oxygen sensors. The sensors have heating elements that reduce the amount of time it takes for the sensors to reach operating temperature.
Removal & Installation
- Disconnect the negative battery cable.
- Raise and support the vehicle.
- Disconnect the electrical connector.
Use a special socket or crows foot wrench to remove the sensor.
NOTEWhen the sensor is removed, the exhaust manifold threads must be cleaned with an 18 mm X 1.5 + 6E tap. If using the original sensor, coat the threads with Loctite 771-64 anti-seize compound or equivalent.
- Install sensor and tighten to 27 Nm (20 ft. lbs.).
- Connect the electrical connector.
- Lower vehicle.
Install the negative battery cable.
WARNINGDo NOT pull on the oxygen sensor wire when unplugging the electrical connector.
CAUTIONThe exhaust manifold and catalytic converter may be extremely hot. Use care when servicing the oxygen sensor.
Before Testing any electrical component, inspect the wiring and connectors for damage. Also wiggle the connectors to ensure a that they are firmly engaged.
- Disconnect the electrical harness from each of the sensors.
- The white wires in the sensor connector are the power and ground circuits for the heater.
- Connect the ohmmeter test leads to the terminals of the white wires in the heated oxygen sensor connector.
- Check the resistance of the sensor, if it is not within 4-7 ohms, replace the sensor.
- Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
- Backprobe with a high impedance averaging voltmeter set to the DC voltage scale.
- Backprobe between the HO2S sensor signal wire and battery ground.
- Verify that the sensor voltage fluctuates rapidly between 0.40-0.60 volts.
- If the sensor voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the voltage fluctuates very slowly within the specified range (H02S signal crosses 0.5 volts less than 5 times in ten seconds), the sensor may be faulty.
- If the sensor voltage stabilizes at either end of the specified range, the PCM is probably not able to compensate for a mechanical problem such as a vacuum leak. These types of mechanical problems will cause the sensor to report a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the H02S sensor test repeated.
- Pull a vacuum hose located after the throttle plate. Voltage should drop to approximately 0.12 volts (while still fluctuating rapidly). This tests the ability of the sensor to detect a lean mixture condition. Reattach the vacuum hose.
- Richen the mixture using a propane enrichment tool. Sensor voltage should rise to approximately 0.90 volts (while still fluctuating rapidly). This tests the ability of the sensor to detect a rich mixture condition.
If the sensor voltage is above or below the specified range, the sensor and/or the sensor wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.
NOTEFurther sensor operational Testing requires the use of a special tester DRB scan tool or equivalent.