GM Astro/Safari 1985-1996 Repair Guide

Oxygen Sensor

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OPERATION



See Figure 1



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Fig. Fig. 1: Heated oxygen sensor components

There are two types of oxygen sensors used in these vehicles. They are the single wire oxygen sensor (O 2 S) and the heated oxygen sensor (HO 2 S). The oxygen sensor is a spark plug shaped device that is screwed into the exhaust manifold. It monitors the oxygen content of the exhaust gases and sends a voltage signal to the Electronic Control Module (ECM). The ECM monitors this voltage and, depending on the value of the received signal, changes the injection parameters to maintain a proper air/fuel ratio. The 1996 models use 4 oxygen sensors. Two are used as inputs to adjust the mixture while the other two are used to monitor the condition of the catalytic converter.

The heated oxygen sensor has a heating element incorporated into the sensor to aid in the warm up to the proper operating temperature and to maintain that temperature.

The proper operation of the oxygen sensor depends upon four basic conditions:

  1. Good electrical connections. Since the sensor generates low currents, good clean electrical connections at the sensor are a must.
  2.  
  3. Outside air supply. Air must circulate to the internal portion of the sensor. When servicing the sensor, do not restrict the air passages.
  4.  
  5. Proper operating temperatures. The ECM will not recognize the sensor's signals until the sensor reaches approximately 600°F (316°C).
  6.  
  7. Non-leaded fuel. The use of leaded gasoline will damage the sensor very quickly.
  8.  

TESTING



Single Wire Sensor
  1. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
  2.  
  3. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (O 2 S) and battery ground.
  4.  
  5. Verify that the O 2 S voltage fluctuates rapidly between 0.40-0.60 volts.
  6.  
  7. If the O 2 S voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the O 2 S voltage fluctuates very slowly between the specified range (O 2 S signal crosses 0.5 volts less than 5 times in ten seconds), the O 2 S may be faulty.
  8.  
  9. If the O 2 S voltage stabilizes at either end of the specified range, the ECM 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 O 2 S to sense a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the O 2 S test repeated.
  10.  
  11. 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 O 2 S to detect a lean mixture condition. Reattach the vacuum hose.
  12.  
  13. 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 O 2 S to detect a rich mixture condition.
  14.  
  15. If the O 2 S voltage is above or below the specified range, the O 2 S and/or the O 2 S wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.
  16.  

Heated Oxygen Sensor
  1. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
  2.  
  3. Turn the ignition OFF disengage the HO 2 S harness connector.
  4.  
  5. Connect a test light between harness terminals A and B on three wire sensors or C and D on four wire sensors. 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 HO 2 S heater or the ground circuit of the HO 2 S heater is faulty. Check the HO 2 S wiring and the fuse.
  6.  
  7. Next, connect a high impedance ohmmeter between the HO 2 S terminals B and Aon 3 wire sensors or C and D on four wire sensors. Verify that the resistance is 3.5-14.0 ohms.
  8.  
  9. If the HO 2 S heater resistance is not as specified, the HO 2 S may be faulty.
  10.  
  11. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
  12.  
  13. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (O 2 S) and battery ground.
  14.  
  15. Verify that the O 2 S voltage fluctuates rapidly between 0.40-0.60 volts.
  16.  
  17. If the O 2 S voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the O 2 S voltage fluctuates very slowly between the specified range (O 2 S signal crosses 0.5 volts less than 5 times in ten seconds), the O 2 S may be faulty.
  18.  
  19. If the O 2 S voltage stabilizes at either end of the specified range, the ECM 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 O 2 S to sense a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the O 2 S test repeated.
  20.  
  21. 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 O 2 S to detect a lean mixture condition. Reattach the vacuum hose.
  22.  
  23. 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 O 2 S to detect a rich mixture condition.
  24.  
  25. If the O 2 S voltage is above or below the specified range, the O 2 S and/or the O 2 S wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.
  26.  

REMOVAL & INSTALLATION



See Figures 2, 3, 4, 5, 6 and 7



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Fig. Fig. 2: Oxygen sensor mounting-2.5L engine



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Fig. Fig. 3: Oxygen sensor mounting (common location)-4.3L engine except 1996 models



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Fig. Fig. 4: Oxygen sensor mounting (alternative location)-4.3L engine except 1996 models



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Fig. Fig. 5: Oxygen sensor mounting locations-1996 4.3L engine



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Fig. Fig. 6: This oxygen sensor wrench has a cut out to allow the wires to pass through



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Fig. Fig. 7: Do not contaminate the tip of the oxygen sensor or the accuracy of the sensor will be affected


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 could 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. Engage the electrical connector.
  4.  
  5. Connect the negative battery cable.
  6.  

 
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