Nissan Sentra/Pulsar/NX 1982-1996 Repair Guide

Oxygen Sensor (O

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OPERATION



See Figures 1 and 2



Click image to see an enlarged view

Fig. Fig. 1: Non-heated type oxygen sensor connector



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Fig. Fig. 2: A three-wire connector indicates that the oxygen sensor is a heated type

The oxygen sensor (O 2 sensor) is a ceramic zirconia bulb which produces an electrical voltage when exposed to the oxygen. The sensor is mounted in the exhaust manifold and is used to detect the amount of oxygen present in the exhaust stream. When there is a large amount of oxygen present (lean mixture), the sensor produces a low voltage. When there is a lesser amount present (rich mixture) it produces a higher voltage.

The O 2 S must reach an operating temperature of approximately 600°F (315°C), in order to function properly. Some sensors contain an electrical heating element to speed up the warm-up time during cold engine operation, and to ensure the sensor remains functional during all phases of engine operation. Sensors using an internal heater are commonly referred to as Heated Oxygen Sensors or HO 2 S.

Front Mounted Sensors

The ECM uses the signal received from the front mounted oxygen sensor, to maintain an ideal fuel mixture of 14.7:1. Since the oxygen sensor is able to rapidly detect oxygen changes in the exhaust and convert it to electrical voltage, the sensor acts as a rich-lean switch. The ECM continually monitors the oxygen sensor's signal and corrects the air/fuel mixture accordingly, thus maintaining the optimum air/fuel mixture.

Rear Mounted Sensors

Beginning in 1995, the GA16DE and SR20DE engines use a second oxygen sensor, mounted after the TWC converter assembly. This sensor, known as a Rear HO 2 S, uses the same operating principles as front mounted sensors mounted. The major difference between the sensors is the purpose they serve. Unlike front mounted sensors, rear mounted sensors have no effect on the fuel management system. The On Board Diagnostic (OBD) system uses the signal received from the rear mounted sensor is to detect the efficiency of the TWC converter.

TESTING



Performance Test
FRONT MOUNTED SENSOR
  1. Set the ECM to Mode II .
  2.  
  3. Start the engine and warm it up until the engine coolant indicator points in the middle of the gauge.
  4.  
  5. Run the engine at about 2000 rpm for 2 minutes under a no-load condition.
  6.  
  7. Observe the Green LED on 1987-90 vehicles, the Red LED on 1991-94 vehicles, or the instrument panel Malfunction Indicator Lamp (MIL) on 1995-96 vehicles. Make sure the lamp, cycles between ON (Lean) and OFF (Rich) more than 5 times every 10 seconds, measured at 2000 rpm under no-load conditions.

    If the lamp operation was as specified, the sensor is operating properly.
     
    If the sensor did not perform as specified proceed to the next step.
     

  8.  

  1. Recheck the harness connections and the ECM pin terminals for damage. Reconnect the ECM harness and repeat the test. If the test results did not change proceed to the next step.
  2.  

Before condemning the sensor, it is important to determine whether the sensor is defective or a fault somewhere else, is causing an improper air/fuel mixture.

  1. If the light is fixed ON , the fuel mixture could be too lean, causing the sensor to continuously produce a lean signal. Before replacing the sensor, verify no other fault codes exist and check the following items:

    Vacuum leaks
     
    Exhaust leaks, before the sensor
     
    Engine Misfire
     

  2.  

  1. If the light is fixed OFF , the fuel mixture could be too rich, causing the sensor to continuously produce a rich signal. Before replacing the sensor, verify no other fault codes exist and check the following items:

    Leaking injectors
     
    Fuel return system restriction
     
    EVAP System fault
     

  2.  

REAR MOUNTED SENSOR

See Figure 3



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Fig. Fig. 3: Rear heated oxygen sensor test points

  1. Connect a voltmeter between ECM terminals 52 (sensor signal) and 43 (engine ground).
  2.  

Do NOT disconnect the connector from the ECM.

  1. Start the engine and allow it to reach normal operating temperature.
  2.  
  3. While observing the voltmeter, rev-up the engine, under no load, between idle speed and 4000 rpm, at least 10 times.
  4.  

To achieve best results, depress and release the accelerator as quickly as possible.



If the sensor remained below 0.55 volts and went above 0.6 volts at least once during the test, the sensor is functioning properly.
 
If the sensor voltage remained above 0.6 volts and did not go below 0.55 volts, proceed to the next step.
 

  1. While observing the voltmeter, perform one or more of the following tests to attempt to determine the sensors low range operation.
    1. Allow the engine to idle for 10 minutes.
    2.  
    3. Roadtest the vehicle. When vehicle is coasting, in 3rd gear, at 50 mph (80 km/h).
    4.  
    5. Rev-up the engine, under no load, between idle speed and 6000 rpms.
    6.  
    7. The voltmeter should have dropped below 0.55 volts at least once during testing.

      If the sensor performed as specified, the Rear HO 2 S is functioning properly.
       
      If the sensor did not perform as specified, replace the Rear HO 2 S and retest.
       

    8.  

  2.  

Heater Test
1991-94 VEHICLES

See Figure 4



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Fig. Fig. 4: Heated oxygen sensor terminal identification-1991-94 vehicles

1991-93 GA16DE engines do not use a heated oxygen sensor.

  1. Disconnect the HO 2 S connector.
  2.  
  3. Measure the resistance across the heater terminals A and C .
  4.  


WARNING
Make certain to properly identify the oxygen sensor terminals, before attempting to test the sensor. Testing across the wrong terminals can damage the sensor.

  1. Resistance should be 3-1000 ohms.
  2.  
  3. If resistance is not as specified, replace the HO 2 S.
  4.  

1995-96 VEHICLES

See Figure 5



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Fig. Fig. 5: Heated oxygen sensor terminal identification-1995-96 vehicles

  1. Disconnect the HO2S connector.
  2.  
  3. Measure the resistance across the heater terminals 1 and 3 .
  4.  


WARNING
Make certain to properly identify the oxygen sensor terminals, before attempting to test the sensor. Testing across the wrong terminals can damage the sensor.

  1. Resistance should be as follows:

    Front sensor-3.3-6.3 ohms at 77°F (25° C)
     
    Rear sensor-5.2-8.2 ohms at 77°F (25° C)
     

  2.  
  3. If resistance is not as specified, replace the HO 2 S.
  4.  

REMOVAL and INSTALLATION

See Figure 6



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Fig. Fig. 6: The oxygen sensor can be removed using a wrench or a special socket

  1. Disconnect the electrical connector and remove the O2 sensor from the exhaust manifold using the proper socket.
  2.  

The oxygen sensor will be easier to remove when the exhaust manifold is warm.

  1. Install the O2 sensor in the exhaust manifold. Torque the sensor to 30-37 ft. lbs. (40-50 Nm).
  2.  
  3. Connect the electrical connector and make sure the wire does not touch any part of the exhaust system.
  4.  

 
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