Toyota Previa 1991-1997 Repair Information

Oxygen Sensor

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OPERATION



The exhaust oxygen sensor or O2S, is mounted in the exhaust stream where it monitors oxygen content in the exhaust gas. The oxygen content in the exhaust is a measure of the air/fuel mixture going into the engine. The oxygen in the exhaust reacts with the oxygen sensor to produce a voltage which is read by the ECM.

There are two types of oxygen sensors 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 pipes. 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, 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:



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

TESTING




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

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 (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 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 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.
  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

See Figure 1

  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 H02S harness connector.
  4.  
  5. Connect a test light between harness terminals +B and HT. 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 H02S heater or the ground circuit of the H02S heater is faulty. Check the H02S wiring and the fuse.
  6.  
  7. Next, connect a high impedance ohmmeter between the H02S terminals of the heating element and verify that the resistance is 11.0-16.0 ohms at 68° F (20° C).
  8.  



Click image to see an enlarged view

Fig. Fig. 1: Using an ohmmeter, measure the resistance between the terminals of the oxygen sensor

  1. If the H02S heater resistance is not as specified, the H02S may be faulty.
  2.  
  3. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
  4.  
  5. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (02S) signal wire and battery ground.
  6.  
  7. Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.
  8.  
  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.
  10.  
  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 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.
  12.  
  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.
  14.  
  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.
  16.  
  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.
  18.  

REMOVAL & INSTALLATION



See Figures 2 through 7

The oxygen sensor can be located in the front pipe/catalytic converter. There are two sensors in the pipe, the rear one is always the sub-sensor. The vehicle must be raised and supported in the air in most situations to remove the sensor(s).



Click image to see an enlarged view

Fig. Fig. 2: There are two oxygen sensors located on either side of the catalytic converter


WARNING
Care should be used during the removal of the oxygen sensor. Both the sensor and its wire can be easily damaged.

  1. The best condition in which to remove the sensor is when the engine is moderately warm. This is generally achieved after two to five minutes (depending on outside temperature) of running after a cold start. Wearing heat resistant gloves is highly recommended during this repair.
  2.  

  1. With the ignition OFF , unplug the connector for the sensor.
  2.  



Click image to see an enlarged view

Fig. Fig. 3: Disconnecting the sensor wiring harness

  1. Remove the two sensor attaching nuts.
  2.  



Click image to see an enlarged view

Fig. Fig. 4: Sometimes it may be hard to access the retaining nuts retaining the sensors



Click image to see an enlarged view

Fig. Fig. 5: Inspect the old nuts, and replace them if necessary

  1. Remove the oxygen sensor from its mount and discard the gasket.
  2.  



Click image to see an enlarged view

Fig. Fig. 6: Remove the sensor ...



Click image to see an enlarged view

Fig. Fig. 7: ... and the old gasket

To install:

During and after the removal, use great care to protect the tip of the sensor if it is to be reused. Do not allow it to come in contact with fluids or dirt. Do not attempt to clean it or wash it.

  1. Apply a coat of anti-seize compound to the bolt threads but DO NOT allow any to get on the tip of the sensor.
  2.  
  3. Position a new gasket, install and secure the sensor to 14 ft. lbs. (20 Nm).
  4.  
  5. Reattach the electrical wiring and insure a clean, tight connection.
  6.  

 
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