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    GM Cavalier/Sunfire 1995-2000 Repair Guide

    Oxygen Sensor

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    OPERATION



    See Figure 1



    Click image to see an enlarged view

    Fig. Fig. 1: Front O2 sensor mounting-2.4L engine

    There are two types of oxygen sensor's used in these vehicles. They are the single wire oxygen sensor (02S) and the heated oxygen sensor (H02S). The oxygen sensor (O2) is a device which produces an electrical voltage when exposed to the oxygen present in the exhaust gases. The sensor is mounted in the exhaust system, usually in the manifold or a boss located on the down pipe before the catalyst.. Some of the oxygen sensors used on the GM J-body 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.

    Some models have two sensors, one before the catalyst and one after. This is done for a catalyst efficiency monitor that is a part of the OBD-II engine controls that are on 1996-00 year vehicles. 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.

    A heated oxygen sensor (HO2S) has a heating element that keeps the sensor at proper operating temperature during all operating modes. Maintaining correct sensor temperature at all times allows the system to enter into CLOSED LOOP operation sooner.

    In CLOSED LOOP operation the PCM monitors the sensor input (along with other inputs) and adjusts the injector pulse width accordingly. During OPEN LOOP operation the PCM ignores the sensor input and adjusts the injector pulse to a preprogrammed value based on other inputs.

    TESTING



    Single Wire Sensor

    See Figure 2



    Click image to see an enlarged view

    Fig. Fig. 2: Inspect the oxygen sensor tip for abnormal deposits

    1. Perform a visual inspection on the sensor as follows:
      1. Remove the sensor from the exhaust.
      2.  
      3. If the sensor tip has a black/sooty deposit, this may indicate a rich fuel mixture.
      4.  
      5. If the sensor tip has a white gritty deposit, this may indicate an internal anti-freeze leak.
      6.  
      7. If the sensor tip has a brown deposit, this could indicate oil consumption.
      8.  

    2.  

    All these contaminates can destroy the sensor, if the problem is not repaired the new sensor will also be damaged.

    1. Reinstall the sensor.
    2.  
    3. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
    4.  
    5. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (02S) and battery ground.
    6.  
    7. Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.
    8.  
    9. If the 02S voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the 02S voltage fluctuates very slowly between the specified range (02S signal crosses 0.5 volts less than 5 times in ten seconds), the 02S may be faulty.
    10.  
    11. If the 02S 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 or a faulty pressure regulator. These types of mechanical problems will cause the 02S to sense a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the 02S test repeated.
    12.  
    13. 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 02S to detect a lean mixture condition. Reattach the vacuum hose.
    14.  
    15. Richen the mixture using a propane enrichment tool. Voltage should rise to approximately 0.90 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a rich mixture condition.
    16.  
    17. If the 02S voltage is above or below the specified range, the 02S and/or the O2S wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.
    18.  

    Heated Oxygen Sensor

    See Figures 3 and 4



    Click image to see an enlarged view

    Fig. Fig. 3: Typical heated oxygen sensor diagram


    WARNING
    Do not pierce the wires when testing this sensor; this can lead to wiring harness damage. Backprobe the connector to properly read the voltage of the HO2S.

    1. Perform a visual inspection on the sensor as follows:
      1. Remove the sensor from the exhaust.
      2.  
      3. If the sensor tip has a black/sooty deposit, this may indicate a rich fuel mixture.
      4.  
      5. If the sensor tip has a white gritty deposit, this may indicate an internal anti-freeze leak.
      6.  
      7. If the sensor tip has a brown deposit, this could indicate oil consumption.
      8.  

    2.  

    All these contaminates can destroy the sensor, if the problem is not repaired the new sensor will also be damaged.

    1. Reinstall the sensor.
    2.  
    3. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
    4.  
    5. Turn the ignition OFF and disengage the H02S harness connector.
    6.  
    7. Connect a test light between harness terminals A and B (refer to graphic). With the ignition switch ON and the engine off, verify that the test light is lit. If the test light is not lit, either the supply voltage to the H02S heater or the ground circuit of the H02S heater is faulty. Check the H02S wiring and the fuse.
    8.  
    9. Next, connect a high impedance ohmmeter between the H02S terminals B and A (refer to graphic) and verify that the resistance is 3.5-14.0 ohms.
    10.  
    11. If the H02S heater resistance is not as specified, the H02S may be faulty.
    12.  
    13. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
    14.  
    15. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (02S) and battery ground.
    16.  
    17. Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.
    18.  
    19. If the 02S voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the 02S voltage fluctuates very slowly between the specified range (02S signal crosses 0.5 volts less than 5 times in ten seconds), the 02S may be faulty.
    20.  
    21. If the 02S 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 or a faulty fuel pressure regulator. These types of mechanical problems will cause the 02S to sense a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the 02S test repeated.
    22.  
    23. 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 02S to detect a lean mixture condition. Reattach the vacuum hose.
    24.  
    25. Richen the mixture using a propane enrichment tool. Voltage should rise to approximately 0.90 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a rich mixture condition.
    26.  
    27. If the 02S voltage is above or below the specified range, the 02S and/or the O2S wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.
    28.  



    Click image to see an enlarged view

    Fig. Fig. 4: The HO2S can be monitored with an appropriate and Data-stream capable scan tool

    REMOVAL & INSTALLATION



    See Figures 5, 6, 7 and 8



    Click image to see an enlarged view

    Fig. Fig. 5: Detach the connector on the oxygen sensor



    Click image to see an enlarged view

    Fig. Fig. 6: A special socket is available to remove the oxygen sensor. the socket contains a slot that the wire slides out of



    Click image to see an enlarged view

    Fig. Fig. 7: Loosen the sensor and ...



    Click image to see an enlarged view

    Fig. Fig. 8: ... remove it from the exhaust manifold/pipe


    WARNING
    The sensor uses a permanently attached pigtail and connector. This pigtail should not be removed from the sensor. Damage or removal of the pigtail or connector will affect the proper operation of the sensor. Keep the electrical connector and louvered end of the sensor clean and free of grease. NEVER use cleaning solvents of any type on the sensor! The oxygen sensor may be difficult to remove when the temperature of the engine is below 120°F (49°C). Excessive force may damage the threads in the exhaust manifold or exhaust pipe.

    1. Disconnect the negative battery cable.
    2.  
    3. Unplug the electrical connector and any attaching hardware.
    4.  
    5. Remove the sensor.
    6.  

    To install:
    1. Coat the threads of the sensor with a GM anti-seize compound, part number 5613695, or its equivalent, before installation. New sensors are precoated with this compound.
    2.  

    The GM anti-seize compound is NOT a conventional anti-seize paste. The use of a regular paste may electrically insulate the sensor, rendering it useless. The threads MUST be coated with the proper electrically conductive anti-seize compound.

    1. Install the sensor and tighten to 30 ft. lbs. (40 Nm). Use care in making sure the silicone boot is in the correct position to avoid melting it during operation.
    2.  
    3. Attach the electrical connector.
    4.  
    5. Connect the negative battery cable.
    6.  

     
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