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.
The heated Oxygen (O 2 S) sensor compares the oxygen content of the surrounding outside air with the quantity of oxygen in the exhaust gasses and produces a corresponding voltage signal as an input to the Engine Control Module (ECM). If the mixture is too rich, a voltage signal indicating low oxygen content is produced ranging from approximately 0.6 to 0.9 volts. If the mixture is too lean, a voltage signal for high oxygen content is produced ranging from about 0.0 to 0.3 volts. During the transition period from rich to lean or lean to rich, the voltage fluctuates between 0.6 to 0.9 volts and 0.0 to 0.3 volts.
Because of the constant and sudden voltage fluctuation, the O 2 S sensor output can correspond to the ideal or stoichiometric mixture, however the mixture control is not held constant, rather the control constantly fluctuates back and forth in a window between the mixture too rich and mixture too lean conditions. When the engine is at operating temperature, the control frequency during idle is a minimum of 30 cycles per minute (0.5 Hz.). As the engine speed is increased to 2500-2800 RPM, the control frequency must be at least 60 cycles a minute (l Hz.). A cycle is a voltage swing from the highest value to the lowest value and back to the highest value again.
If the voltage does not fluctuate, or the sensor responds too slowly (lazy), the following conditions are possible causes:
The time interval between cold start (open loop) and O 2 S sensor control (closed-loop), which requires an oxygen sensor temperature of about 5720°F (3000°C), is affected by the following variables:
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.
- Disconnect the heated oxygen sensor harness electrical connector.
- If installed, open the tie wrap.
- Using a suitable tool, remove heated oxygen sensor.
- Installation is in reverse order of removal noting the following:
- 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.
With a properly functioning O 2 S sensor, the actual control learning values can be checked. Various malfunctions such as an intake air leak or faulty fuel injectors can cause a change in the fuel mixture. These malfunctions are recognized by the O 2 S sensor and corrected by the O 2 S sensor control by adjusting the fuel injection time controlled by the ECM. Fuel injection time that deviates from the programmed basic fuel injection time indicates a malfunction and will likely store a Diagnostic Trouble Code (DTC) and may activate the Check Engine/Malfunction Indicator Light (MIL).Fuel Mixture Control
- Turn the ignition switch to the OFF position.
- Disconnect Heated Oxygen sensors electrical connector.
Connect a suitable Digital Multimeter (DMM) to terminal No. 4 and ground of the connector from the O 2 S sensor. Measure the resistance to check for continuity between terminal No. 4 of the O 2 S sensor signal wire at the O 2 S sensor connector and engine ground. If continuity is not present, replace the heated O 2 S sensor.
- If continuity is present, check for a reference voltage between terminal No. 4 and engine ground. Using the 2 volt range of the DMM, switch the ignition ON and measure the voltage. The voltage should be 350-450 mV. If the voltage reading is not within specification, check the wiring from the ECM and the ECM.