Mazda Trucks 1994-1998 Repair Guide

Heated Oxygen Sensors (HO2S)

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OPERATION



See Figure 1

The oxygen sensor supplies the computer with a signal which indicates a rich or lean condition during engine operation. The input information assists the computer in determining the proper air/fuel ratio. A low voltage signal from the sensor indicates too much oxygen in the exhaust (lean condition) and, conversely, a high voltage signal indicates too little oxygen in the exhaust (rich condition).



Click image to see an enlarged view

Fig. Fig. 1: Typical Heated Oxygen Sensor (H2OS)-most vehicles are equipped with multiple sensors

The oxygen sensors are threaded into the exhaust manifold and/or exhaust pipes on all vehicles. Heated oxygen sensors are used on all models to allow the engine to reach the closed loop faster.

TESTING



B Series Pick-up and Navajo Models

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

  1. Disconnect the HO2S.
  2.  
  3. Measure the resistance between PWR and GND terminals of the sensor. If the reading is approximately 6 ohms at 68°F (20°C). the sensor's heater element is in good condition.
  4.  
  5. With the HO2S connected and engine running, measure the voltage with a Digital Volt-Ohmmeter (DVOM) between terminals HO2S and SIG RTN (GND) of the oxygen sensor connector. If the voltage readings are swinging rapidly between 0.01-1.1 volts, the sensor is probably okay.
  6.  

MPV Models w/2.6L Engine

See Figure 2

MPV models equipped with the 2.6L engine do not have Heated Oxygen Sensors (HO2S), but instead, are equipped with Oxygen Sensors (O2S), which do not contain an integral heating element.


WARNING
Do not pierce the wires when testing this sensor; this can lead to wiring harness damage. Back probe the connector to properly read the voltage of the O2S.

  1. Warm up the engine and allow it to run at idle.
  2.  
  3. Disconnect the O2S.
  4.  



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Fig. Fig. 2: Inspection of the O2S terminal voltage

  1. Connect a voltmeter between the O2S and a good ground.
  2.  
  3. Run the engine at 4,500 rpm until the voltmeter reading indicates approximately 0.7V.
  4.  
  5. Increase and decrease the engine speed suddenly several times. Check to see that when the speed is increased, the voltmeter reads between 0.5V-1.0V and when the speed is decreased, the reading is between 0V-0.4V.
  6.  
  7. If the actual readings are out of specifications, replace the O2S.
  8.  

MPV Models w/3.0L Engine
1994-95

See Figures 3 and 4


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

  1. Warm up the engine and allow it to run at idle.
  2.  
  3. Disconnect the HO2S.
  4.  



Click image to see an enlarged view

Fig. Fig. 3: Inspection of the HO2S terminal voltage

  1. Connect a voltmeter between the HO2S connector terminals A and B on the sensor side.
  2.  
  3. Increase and decrease the engine speed suddenly several times. Check to see that when the speed is increased, the voltmeter reads between 0.5V-1.0V and when the speed is decreased, the reading is between 0V-0.4V.
  4.  
  5. If the actual readings are out of specifications, replace the O2S.
  6.  



Click image to see an enlarged view

Fig. Fig. 4: Inspecting the continuity of the HO2S heating element

  1. Inspect the continuity of the HO2S heating element by connecting an ohmmeter between connector terminals D and C on the sensor side.
  2.  
  3. If there is no continuity, replace the HO2S.
  4.  

1996-98

See Figures 5 and 6


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

  1. Connect NGS tester 49-T088-001 and 49-T088-002 scan tools or equivalent to the data link connector-2 located under the driver side dashboard.
  2.  



Click image to see an enlarged view

Fig. Fig. 5: NGS tester scan tool hook up



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Fig. Fig. 6: HO2S connector terminal identifications

  1. Select the "PID/DATA MONITOR AND RECORD" function on the NGS tester display.
  2.  
  3. Select "FHO2S" and "RHO2S" on the NGS tester display. The NGS tester measures and displays the voltage.
  4.  
  5. The specifications should read as follows:

    With the ignition ON, or at idle: 0V-1.0V
     
    At deceleration: 0V-0.5V
     
    At acceleration: 0.5V-1.0V
     

  6.  
  7. If the readings are out of specifications, inspect the intake air, fuel and On-board diagnostic systems.
  8.  
  9. If these systems are working properly, replace the HO2S.
  10.  
  11. Disengage the HO2S wiring connector.
  12.  
  13. Inspect the continuity of the HO2S heating element by connecting an ohmmeter between connector terminals D and C on the sensor side.
  14.  
  15. If the reading is measures approximately 13 ohms at 68°F (20°C). the sensor's heating element is in good condition.
  16.  
  17. If there is no continuity, replace the HO2S.
  18.  

REMOVAL & INSTALLATION



See Figures 7 and 8

1994-95 MPV models equipped with the 2.6L engine use 1 Oxygen Sensor (O2S). 1994-95 MPV models equipped with the 3.0L engine use 1 Heated Oxygen Sensor (HO2S). These sensors are located on the exhaust pipe between the exhaust manifold and the catalytic converter. 1996-98 MPV models use 2 HO2 sensors. V6 equipped B Series Pick-up models use three HO2S-s, and 2.3L engines use two HO2S-s for the engine control system. The heated sensors are located before and after the dual converters in the exhaust pipes. On 1994-95 V6 engines, there are two sensors, one is located in the left exhaust manifold and the other in the dual converter Y-pipe. 1994-95 2.3L engines use only one sensor.

  1. Disconnect the negative battery cable.
  2.  
  3. Raise and safely support the vehicle on jackstands.
  4.  



Click image to see an enlarged view

Fig. Fig. 7:Oxygen Sensor

  1. Disconnect the HO2S from the engine control sensor wiring.
  2.  

If excessive force is needed to remove the sensors, lubricate the sensor with penetrating oil prior to removal.



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Fig. Fig. 8: Using the proper wrench, loosen the oxygen sensor. Ensure that the wire and connector are unplugged

  1. Remove the sensors with a sensor removal tool, such as Ford Tool T94P-9472-A.
  2.  

To install:
  1. Install the sensor in the mounting boss, then tighten it to 27-33 ft. lbs. (37-45 Nm).
  2.  
  3. Reattach the sensor electrical wiring connector to the engine wiring harness.
  4.  
  5. Lower the vehicle.
  6.  
  7. Connect the negative battery cable.
  8.  

 
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