VW Passat & Audi A4 1990-2000

Oxygen sensor



Do NOT use sealants containing silicone to seal the intake area on engines with oxygen sensors. Silicone particles are not consumed during combustion, consequently the unburned particles travel via the exhaust stream to the oxygen sensor where they can eventually coat the sensor probe and interrupt or destroy proper oxygen sensor function. Do NOT use electrical contact cleaner (or equivalent) in the area of the heated oxygen sensor harness connector because it can lead to corrosion damage of the oxygen sensor.

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:

Slits or holes in the oxygen sensor tip are plugged
sensor was thermally overloaded
sensor damaged via silicone contamination (various sprays)
sensor was lead poisoned with fuel containing lead. Check for lead content using a "PLUMBTESMO" test strip
Terminal resistance in the signal wire
Oxygen sensor too cold-oxygen sensor heater does not function and sensor does not reach ideal operating temperature
Oxygen sensor control switched off (fuel injection system malfunctions stored in DTC memory)
Oxygen sensor damaged with electrical contact cleaner (or equivalent). The spray is drawn in through the signal wire via capillary action resulting from pressure differential and temperature fluctuation and ends up coating the sensor element and air passages causing reduced sensitivity or control failure.

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:

Ambient air temperature (summer/winter)
Operating conditions after start
Proper operation of the O 2 S sensor heater


When performing repairs, Do NOT use any sealants that contain silicone to seal the intake area of an engine using Oxygen sensors. The silicone particles will not be consumed during combustion, thus the unburned particles will travel in the exhaust flow to the O 2 sensor. These particles can ultimately coat the sensor probe(s) and prevent or permanently damage the O 2 sensor operation. Additionally, Do NOT use electrical contact cleaner (or its equivalent) in the area of the HO 2 sensor harness electrical connector(s) because it could lead to corrosion damage of the sensor.

If available, the first step in performing component testing should be to check for any stored Diagnostic Trouble Codes (DTC's) using a VAG 1551, VAG 1552, or suitable scan tool.

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
  1. Turn the ignition switch to the OFF position.
  3. 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.

  1. 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.

Heater Circuit

To check the heated circuit of the O 2 S sensor:

  1. With the ignition OFF, check the resistance between terminal Nos. 1 and 2 of the O 2 S sensor connector. At room temperature the resistance should be 0.0-24.2 ohms.
  3. With the ignition OFF and the O 2 S sensor disconnected, use a suitable DMM to check the voltage between terminal Nos. 1 (+) and 2 (-). Start the engine and allow the engine to idle. The reading should be about 12-14 volts. If the voltage is not present, check the fuses and electrical wiring.

Click image to see an enlarged view

Fig. The Heated Oxygen sensor electrical connector and terminal positions

Click image to see an enlarged view

Fig. The ECM harness Heated Oxygen sensor electrical connector and terminal positions

  1. Disconnect the heated oxygen sensor harness electrical connector.
  3. If installed, open the tie wrap.
  5. Using a suitable tool, remove heated oxygen sensor.
  7. Installation is in reverse order of removal noting the following:
  9. Install all tie wraps in their original locations, to prevent the wiring from contacting exhaust system
  11. Apply a light coating of an anti-seizing compound to the O 2 S threads. DO NOT allow the compound to enter sensor slots
  13. Tighten to: 37ft. lbs. (50 Nm)