The exhaust Oxygen (O2) Sensor is mounted in the exhaust system, ahead of the catalytic converters, where it can monitor the oxygen content of the exhaust gas stream. There are two types of Oxygen sensors in use today. They are the single wire Oxygen Sensor (O2S) and the Heated Oxygen Sensor (HO2S), usually a three or four wire lead. The PCM constantly monitors the O2 signal during closed loop operation and compensates for a rich or lean condition by decreasing or increasing injector pulse width as necessary. The PCM will not go into "closed loop" operation if it can't read the O2 sensor.
Oxygen Sensors generate a voltage signal dependant upon the amount of oxygen reaching the element. This voltage should constantly fluctuate from approximately 1 volt (1000 mV), when reading a rich exhaust, to .01 (100mV) volt if the exhaust is lean. Oxygen sensors can be monitored using a scan tool. By monitoring the voltage output of the oxygen sensor, the PCM calculates what fuel mixture command (pulse width) to give to the injectors. The sensor is like an open circuit and produces no voltage when it is below about 360°C (600°F).
This type oxygen sensor is found on the earlier model vehicles. Later years went to a heated O2 sensor because it would react quicker, allowing the PCM to switch into closed loop operation faster.
- Visually inspect the pigtail for proper routing and connection.
- Check for an adequate air supply, and a clean unclogged air filter.
- Poor PCM to engine block grounds.
- Fuel Injectors; faulty or sticky fuel injectors can cause a false reading or false DTC indicating an O2 sensor problem.
- Fuel pressure; the system will go lean if pressure is too low. The PCM can compensate for some decrease. Likewise, the system will go rich if pressure is too high. The PCM can compensate for some increase. However, if fuel pressure is not in spec, a DTC may be set.
- Vacuum leaks. Check for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system, and crankcase ventilation system.
- Exhaust leaks. An exhaust leak may cause outside air to enter the exhaust gas stream going past the O2 sensor, causing the system to appear lean. Check for exhaust leaks that may cause a false lean condition.
- Fuel contamination; Water, even in small amounts, can be delivered to the fuel injectors. The water passing through the system can cause a lean exhaust. Excessive alcohol in the fuel can also cause this condition.
- Check the EVAP canister for fuel saturation. If the canister is full of fuel, check the canister control and hoses.
- Check for a leaking fuel pressure regulator diaphragm by checking the vacuum line to the regulator for the presence of fuel.
- An intermittent TP sensor output will cause the system to go rich due to a false indication of engine acceleration.
- Mass Air Flow sensor; disconnect the MAF sensor and see if the condition is corrected. A faulty MAF sensor can give a false reading (lean or rich) to the O2 sensor.
- A faulty Manifold Absolute Pressure (MAP) Sensor can affect O2 operation.
For single wire O2 sensors, check the following:
- Connect a DVOM between the signal wire and ground. With the engine at normal operating temperature and the engine running (1200 to 1500) RPM, the O2 sensor should be generating a fluctuating signal between zero and 1 volt. If the reading stays above or below .5 volts and does not fluctuate, the sensor could be bad. If the sensor is slow in switching, or acts lazy in changing it's voltage reading, it could be bad.
- For heated O2 sensors, check the following:
See Figures 1 and 2
- An internally shorted Heated Oxygen Sensor will indicate voltage output of over 1 volt.
- Unplug the sensor connector and check the resistance between terminals C and D. Resistance values should be about 10-15 ohms at about 80°F. If the resistance is not within specification, the heater is faulty.
- If resistance is within specification, check for battery voltage between connector terminals C and D with the key on, engine off.
- Check the O2 sensor voltage between terminals A and B with the engine off. The voltage should be within the 350-500 mV range. If the voltage reading is off , the sensor is faulty. If the reading is good, run the engine while reading the scale. The voltage should vary between 100-1000 mV (0 to 1 volt).
- If the sensor is operating within specifications, check the circuits to the PCM for continuity.
- If the sensor and circuits are functioning properly, the PCM may be faulty.
REMOVAL AND INSTALLATION
- Disconnect the negative battery cable.
See Figure 3
- Detach the sensors pigtail electrical connector.
- Unscrew the O2 sensor.
See Figure 4
A special anti-seize compound is used on the oxygen sensor threads. The compound consists of liquid graphite and glass beads. The graphite will burn away, but the glass beads will remain, making the sensor easier to remove. Most replacement sensors already have the compound on the threads, however you will need to coat the threads if reusing an old sensor.
- If necessary, coat the sensor threads with the anti-seize compound liberally. GM compound #5613695 or equivalent. This is not a conventional anti-seize paste. It is an electrically conductive compound.
- Screw the sensor into the mounting hole in the exhaust system. Torque to 30 ft. lbs. (41Nm).
- Attach the sensors pigtail connector.
- Connect the negative battery cable.