GM Full-Size Trucks 1988-1998 Repair Information

Oxygen Sensor



The oxygen sensor is a spark plug shaped device that is screwed into the exhaust manifold on V6 and V8 engines, and into the exhaust pipe on the 4.8L. It monitors the oxygen content of the exhaust gases and sends a voltage signal to the ECM. The ECM monitors this voltage and, depending on the value of the received signal, issues a command to the mixture control solenoid on the carburetor to adjust for rich or lean conditions.

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.
  3. Outside air supply. Air must circulate to the internal portion of the sensor. When servicing the sensor, do not restrict the air passages.
  5. Proper operating temperatures. The ECM will not recognize the sensor's signals until the sensor reaches approximately 600°F (316°C).
  7. Non-leaded fuel. The use of leaded gasoline will damage the sensor very quickly.


See Figures 1 and 2

Click image to see an enlarged view

Fig. Fig. 1: Inspect the oxygen sensor tip for abnormal deposits

  1. Perform a visual inspection on the sensor as follows:

Remove the sensor from the exhaust. If the sensor tip has a black/sooty deposit, this may indicate a rich fuel mixture. If the sensor tip has a white gritty deposit, this may indicate an internal anti-freeze leak. If the sensor tip has a brown deposit, this could indicate oil consumption.

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

  1. Reinstall the sensor.
  3. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
  5. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (02S) and battery ground.
  7. Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.
  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.
  11. If the 02S 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 high float level. 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.
  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.
  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.
  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.

Click image to see an enlarged view

Fig. Fig. 2: Oxygen sensor (02S) wiring diagram


See Figures 3, 4 and 5

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.

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Fig. Fig. 3: Oxygen sensor-4.3L engines

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Fig. Fig. 4: Oxygen sensor-5.0L and 5.7L engines

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Fig. Fig. 5: Oxygen sensor-7.4L engines

  1. Unplug the electrical connector and any attaching hardware.
  3. Remove the sensor using an appropriate sized wrench or special socket.

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 usually precoated with this compound.

The GM antiseize 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.
  3. Engage the electrical connector and attaching hardware if used.