2002 Test Groups: 2BMXV04.4LEV (540i), 2BMXT04.4E53 & 2BMXT04.6XHP

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General Description - Response rate of upstream oxygen sensor is monitored by measuring period of lambda control oscillations. See Fig 1-Fig 2 .

Fig 1: Oxygen Sensor Monitoring Structure
Courtesy of BMW OF NORTH AMERICA, INC.

Fig 2: Oxygen Sensor Monitoring Function Flow Chart
Courtesy of BMW OF NORTH AMERICA, INC.
Diagnosis Procedure Of Monitor Sensor (Downstream) - Activity of monitor sensor after reaching operating conditions, is determined by 2 different procedures:
- Oscillation Check (Line Crossing) - If following checks are correct, monitor sensor will be regarded as okay:
  1. Monitor sensor signal (sensor voltage) is equal to or greater than nominal value of TV-correction and voltage increases, if lambda control goes to the lean side, or
  2. Monitor sensor signal (sensor voltage) is less than nominal value of TV-correction and voltage decreases, and if lambda control goes to rich side.
    - Fuel Cut-Off Check - In addition to above mentioned checks, signal behavior of monitor sensor is checked in case of fuel cut-off. Therefore, monitor sensor voltage has to be below a given nominal value in case of fuel cut-off. If monitor sensor detected a defect, a fault code is stored and MIL is illuminated at next driving cycle.
Oxygen Sensor Heater Monitoring (Up & Downstream) General Description - For proper function of oxygen sensor, sensor element must be heated. A non-functioning heater delays sensor readiness for closed loop control and influences emissions. Monitoring function measures heater current for both sensors (voltage drop over a shunt) and heater voltage (heater supply voltage) to calculate sensor heater resistance. Monitoring function is activated once per trip if heater has been switched on for a certain time period and current has stabilized. See Fig 3.

Fig 3: Heater Control & Heater Current Measuring Circuitry
Courtesy of BMW OF NORTH AMERICA, INC.
Oxygen Sensor Circuit Monitoring - Monitoring of electrical faults of sensors upstream and downstream of catalyst-not plausible voltages:
- Voltages exceeding maximum threshold are caused by a short circuit to voltage.
- Voltages falling below minimum threshold are caused by a short circuit of sensor signal or sensor ground to PCM ground. See Fig 4.
  
  Fig 4: Oxygen Sensor Monitoring Function Flow Chart
  Courtesy of BMW OF NORTH AMERICA, INC.
If there is no plausible course of sensor voltage, an open circuit of sensor upstream catalyst can be detected if voltage remains in a specified range after sensor has been heated.