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 low voltage. When a high amount of oxygen is present (caused by a lean air/fuel mixture), the sensor produces a high voltage. Because an accurate voltage signal is only produced if the sensor temperature is above approximately 600°F (315°C), 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 which keeps the engine at its peak efficiency.
See Figures 1, 2 and 3
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 sensors. Refer to the accompanying wiring illustration.
- 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.
See Figures 4, 5, 6, 7, 8 and 9
- 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) 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 between 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.
- Further sensor operational testing requires the use of a special tester DRB III scan tool or equivalent.
REMOVAL & INSTALLATION
See Figures 10 and 11
The 3.3L engine uses two heated oxygen sensors, one on each of the exhaust manifolds. The sensors point toward the engine block.
- Raise the vehicle and support it with jackstands.
- Disengage the HO2S sensor wiring connector.
- Remove the sensor by unscrewing it from the manifold.
- Inspect the threads of the HO2S sensor. Apply an anti-seize compound only if there is none visible on the threads. Be careful not to contaminate the sensor tip with any foreign compounds.
- Install the sensor and tighten to 20 ft. lbs. (28 Nm).
- Engage the wiring connector.
- Carefully lower the vehicle.
See Figures 12 and 13
The 3.5L engine uses two heated oxygen sensors, one on each exhaust manifold.
- To access the sensor on the right manifold, remove the air cleaner tube.
- Disconnect the sensor wiring.
- Using a socket such as Snap On® YA8875 or an equivalent crow foot wrench, remove the sensor.
- After removing the sensor, the exhaust manifold threads must be cleaned with an 18mm x 1.5 + 6E tap. If you are reusing the original sensor, coat the sensor threads with an anti-seize compound such as Loctiite® 771-64 or equivalent. New sensors have a compound on the threads and do not require an additional coating. Tighten the sensor to 20 ft. lbs. (28 Nm).
- Connect the sensor wiring.
- Install the air cleaner tube.
See Figure 14
The sensors are located at the outlet ends of the catalytic converter.
- Raise and support the vehicle.
- Disconnect the wiring harness from the HO2S sensor.
Do not pull directly on the wire going to the sensor, this could damage the wiring.
- Remove the sensor using a crow foot wrench such as tool YA8875 or equivalent.
- Install the sensor and tighten to 22 ft. lbs. (30 Nm).
Additional anti seize should not be applied to the threads of the sensor unless the old sensor being installed.
- Connect the sensor harness.
- Lower the vehicle.