The oxygen (O2) sensor 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 system, usually in the manifold or a boss located on the down pipe before the catalyst. The oxygen sensors used on many VW models are electrically heated internally for faster switching when the engine is started cold. 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 ECU.
- To test the oxygen sensor, do not disconnect the wiring harness but insert a back-probe or other devise such as a paper clip into the rear of the wiring harness at terminal # 4 on 93-99 VW's. This wire is the signal wire.
- Using a Digital Volt Ohm Meter (DVOM) measure the oxygen sensors output by connecting the positive lead of the DVOM to the #4 terminal and the negative lead to a good ground.
- Make sure all testing equipment is clear of any moving parts and then start the vehicle.
- Within a few minutes the oxygen sensor should begin producing voltage. At idle this reading should be between 0.3 and 1.0 volts. The reading should fluctuate!
- If the oxygen sensor output is out of range, you will have to test the sensors heating element. This can be accomplished by turning the engine off and disconnecting the four (4) pin harness.
- Perform a test for battery voltage at terminals one (1) and two (2) of the connector. This once again must be performed with the engine running. The resistance should be approximately 2 ohms.
- If the oxygen sensor signal is not as specified, replace the sensor.
REMOVAL & INSTALLATION
- Disconnect the negative battery cable and place it away from the battery's post.
- Detach the oxygen sensor electrical wiring harness.
- Using a oxygen sensor socket (sold at most parts warehouses) of a closed end wrench of comparable size, loosen the sensor from the exhaust.
- Remove the oxygen sensor from the exhaust.
The sensor is threaded into the catalytic converter or the exhaust pipe with an anti-seize compound on the threads.
- To install simply thread the sensor into the exhaust and tighten.