Oxygen Sensor: Testing and Inspection
Motronic ECU Harness Connector (pin # Locations):
OXYGEN SENSOR HEATING ELEMENT CHECKING
1. Locate oxygen sensor heating element connector (this is the two terminal connector coming off the wire leads to the sensor) and disconnect from harness.
2. Start engine and idle.
3. With engine running, check voltage between terminals on harness side of connector.
- Should be battery voltage. If not, repair open circuit between terminal #2 of harness connector and terminal "B15" of fuse relay panel, or between harness connector terminal #1 and ground.
4. If battery power is present, connect ammeter between harness connector and sensor connector (use appropriate connectors to complete the circuit), set to 10 amp range.
- With engine running, current to the heating element should be between 0.5 amps (hot sensor) and 3.0 amps (cold sensor). If current is greater than 3.0 amps, or less than 0.5 amps, heating element in sensor body is faulty. Replace sensor.
OXYGEN SENSOR CHECKING
1. Check for open or grounded circuit between oxygen sensor connector (single pin connector on end of sensor lead wires) and ECU terminal #7, and make sure wire shielding is not damaged and is connected to ECU terminal #35 (ground).
2. With engine at normal operating temperature, connect parallel test lead between harness side and sensor side of oxygen sensor connector, and connect DVM between test lead and engine ground. Set DVM to 2 volt range.
3. Disconnect electrical connector from differential pressure regulator and install appropriate Audi electrical harness adapter (Audi part #VW 1315 A/1).
Checking Differential Pressure Regulator Current:
4. Connect DVM to adapter, set mode to DC milliamps 200mA scale.
5. Start engine and run at 2,000 rpm for 1 minute, then let idle.
6. Check oxygen sensor output voltage.
- Voltage should be varying between 300-600 millivolts (0.3-0.6 volts).
7. Observe differential pressure regulator current.
- Current should vary between 0 and +5 mA. If not, check basic air/fuel ratio adjustment. Adjustments If current is more than +5 mA, basic adjustment is too lean. If current is negative in polarity, basic adjustment is too rich.
8. Impose a lean condition by creating a small vacuum leak (unplug a vacuum hose) and observe regulator current and oxygen sensor voltage.
- Oxygen sensor voltage should immediately drop (for a moment) and then return to the normal range as the computer compensates for the imposed lean condition. At the same time as the oxygen sensor voltage returns to normal, the pressure regulator current should increase and continue varying by approximately 5 mA, but at a slightly higher value (i.e. current varies between +5 and +10 mA).
If the voltage and current readings respond as described, the oxygen sensor and the mixture control functions of the computer are operating normally. If the regulator current is fixed or becomes fixed during testing, run engine at 2,000 rpm for 1 minute to heat sensor up to operating temperature (300~C/575~F minimum) and re-check. If no voltage reading is obtained from oxygen sensor, replace sensor. If oxygen sensor voltage rises to 0.9-1.0 volts and remains fixed at this voltage, either the engine is running extremely rich (too rich for the computer to compensate), or the oxygen sensor is contaminated and the engine is actually running lean in response to the false rich indication from the sensor. In either case the computer will be sending a "full lean command" to the differential pressure regulator (regulator current will be negative and fixed at its lean limit), and the oxygen sensor should be replaced. If a new sensor's output voltage rises to 0.9-1.0 volts and remains fixed at this point, the engine is running too rich and the rich condition must be corrected or the new sensor may become contaminated with carbon and rendered inoperative.