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    Mercury Villager Nissan Quest 1997-2000

    Heated Oxygen Sensors

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    OPERATION

    The oxygen (O2) sensor is a device which monitors exhaust oxygen content. An electrical voltage within 0-1 volt is produced when the sensor is exposed to the oxygen present in the exhaust gases. 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 oxygen sensor signal voltage is monitored by the PCM.

    The PCM ignores oxygen sensor signals until a predetermined engine temperature is reached. Until that time, the PCM operates in what is known as OPEN LOOP operation. During OPEN LOOP operation, the PCM adjusts the fuel injector pulse width to a preprogrammed value based on other inputs. After the engine reaches normal operating temperature, the PCM will enters what is known as CLOSED LOOP operation. During CLOSED LOOP operation, the PCM monitors oxygen sensor input, along with other inputs, and adjusts the fuel injector pulse width accordingly.

    Heated oxygen sensors (HO2S) have 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.

    1993-95 vehicles have one heated oxygen sensor mounted in the exhaust inlet pipe, which provides the PCM with information to determine fuel injection pulse width. 1996-00 vehicles have two heated oxygen sensors, a front sensor before the catalytic converter, and a rear one after. The front heated oxygen sensor location and function are the same as in earlier models. The rear heated oxygen sensor, mounted between the three way catalytic converter (TWC) and the muffler, provides the PCM with information to determine the difference in the emissions after the exhaust gases have gone through the TWC.

    TESTING
    Front Heated Oxygen Sensor

    Front heated oxygen sensor circuit operation can be monitored with the use of a scan tool, or when the PCM is put in diagnostic mode. Please refer to Trouble Codes, Reading Codes, Oxygen Sensor Monitor for the procedure.

    1. Run the engine until it reaches normal operating temperature.
    2.  
    3. Using a voltmeter set to the DC range, connect the negative lead of the voltmeter to the negative battery terminal or engine ground.
    4.  
    5. Connect the positive voltmeter lead to the back of the PCM electrical connector in the heated oxygen sensor terminal:
    6.  
    7. 1993-95 vehicles: pin 19
    8.  
    9. 1996-98 vehicles: pin 46
    10.  
    11. 1999-00 vehicles: pin 50
    12.  
    13. While holding the engine speed at 2,000 rpm with the transaxle in park, observe the voltmeter reading for ten seconds.
    14.  
    15. The maximum voltage should be over 0.6 volts at least once, and the minimum voltage should be under 0.3 volts at least once. If the voltage is not within specifications, the sensor may be faulty.
    16.  

    Rear Heated Oxygen Sensor
    1. Drive the vehicle at a speed of more than 43 mph (70 km/h) for at least two minutes.
    2.  
    3. Stop the vehicle. Put the transaxle in park and leave the engine running.
    4.  
    5. Using a voltmeter set to the DC range, connect the negative lead of the voltmeter to the negative battery terminal or engine ground.
    6.  
    7. Connect the positive voltmeter lead to the back of the PCM electrical connector in terminal 56.
    8.  
    9. While revving the engine up to 4,000 rpm with the transaxle in park ten times, observe the voltmeter reading. At least once, the reading should be above 0.48 volts on 1996-98 vehicles. On 1999-00 vehicles, the reading should be above 0.56 volts.
    10.  
    11. Allow the engine to idle for ten minutes and observe the voltmeter reading. At least once, the reading should be below 0.43 volts on 1996-98 vehicles. On 1999-00 vehicles, the reading should be below 0.54 volts.
    12.  
    13. If the voltages are not within specifications, the sensor may be faulty.
    14.  

    REMOVAL & INSTALLATION
    Front Heated Oxygen Sensor
    1. Disconnect the negative battery cable.
    2.  
    3. Raise and safely support the front of the vehicle securely on jackstands.
    4.  
    5. Remove the driver's side engine and transaxle splash shield.
    6.  
    7. Disconnect the front heated oxygen sensor electrical connector.
    8.  
    9. Remove the sensor using an appropriate tool. Special sockets for oxygen sensor removal are available from many parts stores and automotive tool suppliers. The proper size wrench may also be used.
    10.  

    To install:

    1. Coat the threads of the sensor with a suitable anti-seize compound. New sensors usually come pre-coated with compound.
    2.  
    3. Install the sensor and tighten it to 37-44 ft. lbs. (50-60 Nm).
    4.  
    5. Connect the oxygen sensor electrical connector.
    6.  
    7. Install the splash shield.
    8.  
    9. Connect the negative battery cable.
    10.  



    Click image to see an enlarged view

    Fig. A special socket is available to remove the oxygen sensor. The socket has a slot that the harness slides out of



    Click image to see an enlarged view

    Fig. The correct size wrench can be used if an oxygen sensor socket is not available

    Rear Heated Oxygen Sensor
    1996-00 vehicles (except for 1998)
    1. Disconnect the negative battery cable.
    2.  
    3. Raise and safely support the front of the vehicle securely on jackstands.
    4.  
    5. Remove the grommet from the floor pan and guide the harness through the opening to gain access to the electrical connector.
    6.  
    7. Disconnect the rear heated oxygen sensor electrical connector.
    8.  
    9. Remove the sensor using an appropriate tool. Special sockets for oxygen sensor removal are available from many parts stores and automotive tool suppliers. The proper size wrench may also be used.
    10.  

    To install:

    1. Coat the threads of the sensor with a suitable anti-seize compound. New sensors usually come pre-coated with compound.
    2.  
    3. Install the sensor and tighten it to 37-44 ft. lbs. (50-60 Nm).
    4.  
    5. Connect the oxygen sensor electrical connector.
    6.  
    7. Connect the negative battery cable.
    8.  



    Click image to see an enlarged view

    Fig. Location of the rear heated oxygen sensor



    Click image to see an enlarged view

    Fig. Remove the grommet from the floor pan to gain access to the rear oxygen sensor electrical connector

    1998 vehicles
    1. Disconnect the negative battery cable.
    2.  
    3. Remove the passenger's side front seat. Please refer to Body and Trim, Interior, Seats, Removal and Installation for the procedure.
    4.  
    5. Remove the passenger's side front scuff plate to gain access to the rear heated oxygen sensor wiring harness and connector. This is the panel on the body at the bottom of the door. There are four retaining clips that hold the plate in place. Pull up on the plate to remove it.
    6.  
    7. Disconnect the rear heated oxygen sensor electrical connector.
    8.  
    9. Remove the grommet from the floor pan and guide the harness through the opening.
    10.  
    11. Raise and safely support the front of the vehicle securely on jackstands.
    12.  
    13. Remove the sensor using an appropriate tool. Special sockets for oxygen sensor removal are available from many parts stores and automotive tool suppliers. The proper size wrench may also be used.
    14.  

    To install:

    1. Coat the threads of the sensor with a suitable anti-seize compound. New sensors usually come pre-coated with compound.
    2.  
    3. Install the sensor and tighten it to 37-44 ft. lbs. (50-60 Nm).
    4.  
    5. Guide the harness up through the floor pan.
    6.  
    7. Lower the vehicle.
    8.  
    9. Install the grommet and connect the oxygen sensor electrical connector.
    10.  
    11. Install the passenger's side front scuff plate.
    12.  
    13. Install the passenger's side front seat.
    14.  
    15. Connect the negative battery cable.
    16.  

     
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