Volkswagen Front Wheel Drive 1974-1989 Repair Guide

Oxygen Sensor


This sensor is used to report the concentration of oxygen in the exhaust. It consists of a tube coated with platinum on the outside and zirconia on the inside. The tube is protected with a slotted outer shield. The platinum side is exposed only to exhaust gas. If there is any oxygen in the exhaust, a voltage is generated across the dissimilar metals that the engine control unit can read. The sensor operates only when it is above about 600°F (315°C). Some sensors include a built-in heater for faster response when the engine is started cold.


An exhaust gas analyzer is required to test the oxygen sensor.

  1. Disconnect the wiring at the sensor. If there is a brown wire, it is ground for the sensor heater. The green wire is for the sensor itself, the other wire is for voltage to the heater. On the wiring harness side of the connector, there should be 12 volts at the heater wire terminal with the ignition switch ON .
  3. Turn the ignition switch OFF and connect an ohmmeter to the heater wire on the sensor connector. There should be 3-15 ohms resistance between the sensor heater and ground.
  5. To test the sensor output, reconnect the wiring and warm the engine to normal operating temperature.
  7. Remove the plastic cap from the exhaust gas sample tap and insert the probe of an exhaust gas analyzer. With the engine at idle, disconnect the fuel pressure regulator vacuum hose from the intake manifold and plug the port. The CO reading should increase momentarily, then return to the original value. If it does not, the sensor or the engine control unit may be faulty.


See Figure 1

The sensor is threaded into the catalytic converter or the exhaust pipe. An anti-seize compound is used on the threads. When replacing it, be careful not to get anti-seize in the slots of the outer shield. Torque to 37 ft. lbs. (50 Nm).

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Fig. Fig. 1: The oxygen sensor may be threaded into the catalytic converter or the exhaust pipe