Chrysler Caravan/Voyager/Town and Country 1996-1999

Heated Oxygen Sensor

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OPERATION



As a vehicle accrues mileage, the catalytic converter deteriorates. The deterioration results in a less effective catalyst. To monitor catalytic converter deterioration, the fuel injection system uses two heated oxygen sensors: one which is upstream of the catalytic converter and one downstream of the converter.

The heated oxygen sensor, or HO2S sensor is usually located near the catalytic converter. It produces a voltage signal of 0.1-1.0 volts based on the amount of oxygen in the exhaust gas. When a low amount of oxygen is present (caused by a rich air/fuel mixture), the sensor produces a high voltage. When a high amount of oxygen is present (caused by a lean air/fuel mixture), the sensor produces a low voltage. Because an accurate voltage signal is only produced if the sensor temperature is above approximately 600°F (315°C), a fast acting heating element is built into its body.

The PCM uses the HO2S sensor voltage signal to constantly adjust the amount of fuel injected which keeps the engine at its peak efficiency.

The PCM compares the reading from the sensors to calculate the catalytic converter oxygen storage capacity and storage efficiency. The PCM also uses the upstream heated oxygen sensor input when adjusting the injector pulse width. When the catalytic converter efficiency drops below preset emission criteria, the PCM stores a Diagnostic Trouble Code (DTC) and illuminates the Malfunction Indicator Lamp (MIL).

The automatic shutdown relay supplies battery voltage to both of the heated oxygen sensors. The sensors have heating elements which reduce the amount of time it takes for the sensors to reach operating temperature.

TESTING



Heating Element

See Figure 1



Click image to see an enlarged view

Fig. Fig. 1: Heated oxygen sensor connector terminal identifications

Before testing any electrical component, inspect the wiring and connectors for damage. Also wiggle the connectors to ensure a that they are firmly engaged.

  1. Disconnect the electrical harness from each of the sensors.
  2.  

The white wires in the sensor connector are the power and ground circuits for the heater.

  1. Connect the ohmmeter test leads to the terminals of the white wires in the heated oxygen sensor connector.
  2.  
  3. Check the resistance of the sensor, if it is not within 4-7 ohms, replace the sensor.
  4.  

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. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the HO2S sensor signal wire and battery ground.
  4.  
  5. Verify that the sensor voltage fluctuates rapidly between 0.40-0.60 volts.
  6.  
  7. If the sensor voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the voltage fluctuates very slowly between the specified range (H02S signal crosses 0.5 volts less than 5 times in ten seconds), the sensor may be faulty.
  8.  
  9. If the sensor voltage stabilizes at either end of the specified range, the PCM is probably not able to compensate for a mechanical problem such as a vacuum leak. These types of mechanical problems will cause the sensor to report a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the H02S sensor 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 sensor to detect a lean mixture condition. Reattach the vacuum hose.
  12.  
  13. Richen the mixture using a propane enrichment tool. Sensor voltage should rise to approximately 0.90 volts (while still fluctuating rapidly). This tests the ability of the sensor to detect a rich mixture condition.
  14.  
  15. If the sensor voltage is above or below the specified range, the sensor and/or the sensor wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.
  16.  
  17. Further sensor operational testing requires the use of a special tester DRB scan tool or equivalent.
  18.  

REMOVAL & INSTALLATION



Upstream Heated Oxygen Sensor

See Figures 2, 3, 4 and 5



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Fig. Fig. 2: The oxygen sensor can be accessed from the top of the vehicle by removing the windshield wiper/motor module assembly



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Fig. Fig. 3: Locate the sensor connector and disengage



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Fig. Fig. 4: Using a special oxygen sensor socket tool, loosen the sensor ...



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Fig. Fig. 5: ... then remove the heated oxygen sensor from the exhaust manifold

  1. Disconnect the negative battery cable.
  2.  

The upstream heated oxygen sensor is accessible from the top of the engine compartment, however, removal of the windshield wiper/motor module assembly is required.

  1. Raise and safely support the vehicle, unless the sensor is going to be removed from the top of the engine compartment.
  2.  
  3. Unplug the upstream oxygen sensor connector.
  4.  
  5. Remove the sensor using a suitable oxygen sensor crow foot wrench or oxygen sensor socket. After removing the sensor, the exhaust manifold must be cleaned with an 18mm x 1.5 + 6E tap.
  6.  

To install:
  1. New oxygen sensors will be packaged with a special anti-seize compound already applied to the threads. If you are reinstalling the old sensor, the sensor threads must be coated with fresh anti-seize compound. You must use Loctite 771-64®, or equivalent, type of anti-seize compound. This is not a conventional anti-seize paste. The use of a regular compound may electrically insulator the sensor, rendering it inoperative. You must coat the threads with an electrically conductive anti-seize compound.
  2.  
  3. Carefully thread the sensor into the bore, then tighten to 20 ft. lbs. (27 Nm).
  4.  
  5. Attach the oxygen sensor electrical connector.
  6.  
  7. If raised, carefully lower the vehicle. If the sensor was removed from up top, install the windshield wiper/motor module assembly.
  8.  
  9. Connect the negative battery cable.
  10.  

Downstream Heated Oxygen Sensor

See Figures 6, 7, 8, 9 and 10



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Fig. Fig. 6: The downstream heated oxygen sensor is located under the vehicle. The sensor connector is on the other side of the rubber grommet



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Fig. Fig. 7: Pry out the grommet from the vehicle's floor and pull down the wiring harness connector ...



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Fig. Fig. 8: ... then disengage the wiring harness connector



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Fig. Fig. 9: An easily accessible oxygen sensor such as this one can either be removed using a special oxygen sensor socket ...



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Fig. Fig. 10: ... or just a regular open end wrench

  1. Disconnect the negative battery cable.
  2.  
  3. Raise and safely support the vehicle.
  4.  
  5. Remove the grommet cover to expose the sensor connector.
  6.  
  7. Unplug the electrical connector from the downstream oxygen sensor by first pushing back the connector locking tab.
  8.  
  9. Remove the sensor using a suitable oxygen sensor crow foot wrench. After removing the sensor, the exhaust manifold must be cleaned with an 18mm x 1.5 + 6E tap.
  10.  

To install:
  1. New oxygen sensors will be packaged with a special anti-seize compound already applied to the threads. If you are reinstalling the old sensor, the sensor threads must be coated with fresh anti-seize compound. You must use Loctite 771-64®, or equivalent, type of anti-seize compound. This is not a conventional anti-seize paste. The use of a regular compound may electrically insulator the sensor, rendering it inoperative. You must coat the threads with an electrically conductive anti-seize compound.
  2.  
  3. Carefully thread the sensor into the bore, then tighten to 20 ft. lbs. (27 Nm).
  4.  
  5. Route the sensor electrical harness through the clips along the body.
  6.  
  7. Attach the oxygen sensor electrical connector.
  8.  
  9. Place the connector back into its original location and install the grommet cover.
  10.  
  11. Carefully lower the vehicle, then connect the negative battery cable.
  12.  

 
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