Subaru Coupes/Sedans/Wagons 1985-1996 Repair Guide

Oxygen Sensor

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OPERATION



See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Cutaway view of a single wire oxygen sensor

There are two types of oxygen sensor's used in these vehicles. They are the single wire oxygen sensor (02S) and the heated oxygen sensor (H02S). 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 (ECU). The ECU 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 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 ECU 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 or multimeter,set to the DC voltage scale, between the oxygen sensor (02S) and battery ground.
  4.  
  5. Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.
  6.  
  7. 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.
  8.  
  9. If the 02S voltage stabilizes at either end of the specified range, the ECU is probably not able to compensate for a mechanical problem such as a vacuum leak, faulty pressure regulator or 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, then the 02S 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 02S 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 02S to detect a rich mixture condition.
  14.  
  15. 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.
  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 , and disengage the H02S harness connector.
  4.  
  5. Check for battery voltage at the wires with the ignition switch ON and the engine off. If not, there is a problem in the wiring. Check the H02S wiring and the fuse.
  6.  
  7. Next, connect a high impedance ohmmeter between the black wire and white wire. Verify that the resistance is 3.5-14.0 ohms.
  8.  
  9. If the H02S heater resistance is not as specified, the H02S 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 or multimeter, set to the DC voltage scale between the oxygen sensor (02S) and battery ground.
  14.  
  15. Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.
  16.  
  17. 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.
  18.  
  19. If the 02S voltage stabilizes at either end of the specified range, the ECU 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 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.
  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 02S 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 02S to detect a rich mixture condition.
  24.  
  25. 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.
  26.  

REMOVAL & INSTALLATION



See Figure 2

Click image to see an enlarged view

Fig. Fig. 2: Loosening the oxygen sensor connection-Legacy


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. Remove the exhaust manifold plate, as required.
  4.  
  5. Unplug the electrical harness.
  6.  
  7. Loosen and remove the sensor.
  8.  

To install:
  1. Coat the threads of the sensor with an anti-seize compound before installation. New sensors are precoated with this compound.
  2.  

DO NOT use 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 into the exhaust manifold and tighten it 18-25 ft. lbs. (24-34 Nm).
  2.  
  3. Install the exhaust manifold plate, as required.
  4.  
  5. Engage the electrical harness.
  6.  
  7. Connect the negative battery cable.
  8.  

 
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