Use a BMW service tester, the OBD I blink code or on OBD II vehicles, use a suitable Data Scan Tool (DST) to check for faults and/or if capable, to read the values of the components. If this diagnostic equipment is not available, proceed as follows.
During normal operation the Oxygen (O 2 ) sensor voltage should continuously fluctuate as follows:
To check the O 2 sensor response to lean mixtures create an air leak by removing a vacuum hose.
To check the O 2 sensor response to a rich mixture, pull the vacuum hose off the fuel pressure regulator to increase fuel pressure.
To check the operation of the Bosch® HO 2 /O 2 sensor proceed as follows:
- Allow the engine to reach normal operating temperature, then turn the engine off.
- Raise and safely support the vehicle.
- Carefully slide the protective boot away from the chassis side HO 2 , O 2 sensor electrical connector and using a digital voltmeter probe pin Nos. 1 and 2 (sensor signal and ground).
- Carefully start the engine. The HO 2 /O 2 sensor voltage should begin to fluctuate within a couple of minutes of operation. If the voltage fails to fluctuate, turn the engine off. On HO 2 sensors, check the operation of the sensor heating circuit. If not equipped with a heater circuit, replace the sensor. On models with a heated sensor, if the sensor fails to produce a fluctuating voltage and heater circuit is OK, replace the sensor.
On vehicles with Heated Oxygen (HO 2 ) sensors check the operation of the heater circuit and element as follows:
- Detach the HO 2 sensor electrical connector and check for battery voltage between terminal Nos. 3 and 4 (the green and brown wires) on the chassis side of the electrical wiring harness while the engine is running. If battery voltage is not present, check the wiring and the operation of the HO 2 sensor heater relay, which is mounted in the fuse panel/power distribution box under the hood toward the left rear. The heater relay is receives positive battery voltage (+) from the main relay and is actuated via a ground signal received from the ECM.
- Check the heater element resistance between terminals 3 and 4 on the sensor side of electrical connector. If no resistance is measured (open circuit), then replace the sensor.