Volvo 240/740/760/780/940/960 1990-1998

Heated Oxygen Sensor

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OPERATION



See Figure 1

The Oxygen Sensor (O2S) is a device which produces an electrical voltage when exposed to the oxygen present in the exhaust gases. The sensor is mounted in the exhaust manifold. Some oxygen sensors are electrically heated internally for faster switching when the engine is running. The oxygen sensor produces a voltage within 0 and 1 volt. When there is a large amount of oxygen present (lean mixture), the sensor produces a low voltage (less than 0.4v). When there is a lesser amount present (rich mixture) it produces a higher voltage (0.6-1.0v).The stoichiometric or correct fuel to air ratio will read between 0.4 and 0.6v. By monitoring the oxygen content and converting it to electrical voltage, the sensor acts as a rich-lean switch. The voltage is transmitted to the engine controller. The controller signals the power module to trigger the fuel injector.

Later models have two sensors, one before the catalytic converter and one after. This is done for a catalyst efficiency monitor that is a part of the OBD-II engine controls that are on these year vehicles. The one before the catalyst measures the exhaust emissions right out of the engine, and sends the signal to the ECM about the state of the mixture as previously talked about. The second sensor reports the difference in the emissions after the exhaust gases have gone through the catalyst. This sensor reports to the ECM the amount of emissions reduction the catalyst is performing.

The oxygen sensor will not work until a predetermined temperature is reached, until this time the engine controller is running in what as known as OPEN LOOP operation. OPEN LOOP means that the engine controller has not yet begun to correct the air-to-fuel ratio by reading the oxygen sensor. After the engine comes to operating temperature, the engine controller will monitor the oxygen sensor and correct the air/fuel ratio from the sensor's readings. This is what is known as CLOSED LOOP operation.

A Heated Oxygen Sensor (HO2S) has a heating element that keeps the sensor at proper operating temperature during all operating modes. Maintaining correct sensor temperature at all times allows the system to enter into CLOSED LOOP operation sooner.

In CLOSED LOOP operation, the engine controller monitors the sensor input (along with other inputs) and adjusts the injector pulse width accordingly. During OPEN LOOP operation, the engine controller ignores the sensor input and adjusts the injector pulse to a preprogrammed value based on other inputs.



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Fig. Fig. 1: OBD-II equipped vehicles have two HO2 sensors, one before and one after the catalyst

TESTING



See Figures 2 and 3


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.

  1. Disconnect the HO2S.
  2.  
  3. Check for supply voltage to the heating element by checking the power and ground circuits using the wiring diagrams located in Chassis Electrical . Check the heating element by:
    1. Turn the ignition to RUN position, with the engine OFF .
    2.  
    3. Using a DVOM, probe the proper circuits connecting the positive lead of the DVOM to the power circuit and the negative lead to the ground circuit.
    4.  
    5. Battery voltage should be present, if voltage is present but less than 8 volts, perform the following step. If there is no voltage skip the next step.
    6.  
    7. Remove the connector on the ECM and check the resistance of the HO2S heaters power and ground circuits, if more than 5 ohms, repair the circuit(s). If resistance is less than 5 ohms, replace the ECM.
    8.  
    9. If no voltage is present check for continuity of the HO2S heaters power and ground circuits. If continuity is present and resistance is less than 5 ohms, replace the ECM, if the circuit is open or resistance exceeds 5 ohms repair the circuit(s).
    10.  

  4.  
  5. Start the vehicle and let it reach operating temperature. With the HO2S connected and engine running, measure the voltage with a Digital Volt-Ohmmeter (DVOM) between terminals HO2S and SIG RTN (GND) of the oxygen sensor connector. Voltage should fluctuate between 0.01-1.1 volts. If voltage fluctuation is slow or voltage is not within specification, the sensor may be faulty.
  6.  



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Fig. Fig. 2: Location of the HO2S connector-700 series



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Fig. Fig. 3: Location of the HO2S connector-240 series

REMOVAL & INSTALLATION



See Figures 4, 5, 6, 7, 8, 9, 10, 11 and 12

  1. Disconnect the negative battery cable.
  2.  
  3. Raise and support the vehicle safely.
  4.  
  5. Label and disconnect the HO2S from the engine control wiring harness.
  6.  

Lubricate the sensor with penetrating oil prior to removal.

  1. Remove the sensor using an appropriate tool. Special oxygen sensor sockets are available to remove the sensor and can be purchased at many parts stores or where automotive tools are sold. The proper size wrench can be used, most sensors are 7 / 8 inch or 22mm sizes.
  2.  

To install:
  1. Before installing, apply Never Seez paste (P/N 1 161 035-9) or equivalent anti-seize compound to the threaded section of the sensor.
  2.  
  3. Install the sensor in the mounting boss and tighten to 40 ft. lbs. (55 Nm).
  4.  
  5. Connect the engine control wiring harness to the sensor.
  6.  
  7. Lower the vehicle.
  8.  
  9. Connect the negative battery cable.
  10.  
  11. Remove the oxygen sensor, using a suitable wrench.
  12.  



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Fig. Fig. 4: Location of the front HO2S on an OBD-II equipped 850 model



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Fig. Fig. 5: The catalyst efficiency monitor HO2S is located under this shield on 850 models



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Fig. Fig. 6: The sensor can be seen from the side, but the panel must first be removed if the sensor is to be removed



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Fig. Fig. 7: Unplug the HO2S connectors



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Fig. Fig. 8: Remove the clip retaining the harness for the HO2S



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Fig. Fig. 9: The rear HO2S has an additional clip retaining the harness



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Fig. Fig. 10: A proper size open end wrench can be used to remove the HO2S



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Fig. Fig. 11: Remove the HO2S from the exhaust pipe



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Fig. Fig. 12: Inspect the oxygen sensor tip for abnormal deposits

 
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