2003 Test Groups: 3BMXV06.0N73

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Heater Diagnosis Of Upstream Wide-Range Oxygen Sensor - Wide range air fuel ratio sensor is ready for operation at temperatures above 1112°F (600°C). In most cases, exhaust gas temperature is not sufficient for heating, and so sensor heating is needed for a proper functioning of sensor. Diagnosis consists of 3 checks:
- Operation Readiness - Sensor readiness depends on heater performance, which is why time delay between "heater on" and "operation readiness" is monitored. Therefore, sensor readiness is checked at a calibrated time after heater has been switched on. If sensor is not ready, a fault code is set.
- Heater Performance In Fuel Cut-Off Operation - A second check is performed during fuel cut-off. During this mode, sensor output voltage is expected to remain within a calibrated range. Otherwise, heater performance is not sufficient and a fault code is set.
- Power Stage Diagnosis - During power stage on and off, control signal (input) of power stage is compared to its output signal. Also, in switched on condition, heater current is checked against a minimum limit. With these checks, wire interruption as well as short to either ground or battery voltage can be detected and, if necessary, fault codes can be set. See Fig 1.
  
  Fig 1: Oxygen Sensor Monitoring Overview
  Courtesy of BMW OF NORTH AMERICA, INC.
Diagnosis of Wide-Range Oxygen Sensor - Diagnosis of upstream oxygen sensor consists of 4 checks, of which some are subdivided into several checks:
- Offset Check - Offset check monitors incorrect lambda measurement due to shunting effects. If lambda offset of downstream-control exceeds a threshold, a fault code is set.
- Heater Coupling Check - Heater coupling check monitors low impedance coupling between heater and sensor, which can cause lambda modulations with heater pulse rate. If difference of consecutive lambda values exceeds calibration, a fault code is set.
- Dynamics Checks (Slow Response) - Due to age, sensor dynamics can decrease. Dynamics check in normal operation mode compares measured and estimated (model-based) lambda behavior, caused by artificial lambda modulation. If ratio of measured and estimated amplitude is below calibration, a fault code is set.
- Plausibility Check
  1. Wide-range oxygen sensor is not active: If lambda value is close to 1.0 for a period of time, a fault code is set. Monitoring condition for plausibility check is if downstream oxygen sensor voltage indicates lean or rich mixture
  2. Sensor current high (lean): If lambda value exceeds calibration, but downstream oxygen sensor indicates rich mixture, a fault code is set.
  3. Sensor current high (rich): If lambda value is below calibration, but downstream oxygen sensor indicates lean mixture, a fault code is set. See Fig 2.
    
    Fig 2: Wide Range Air Fuel Sensor Output Voltage
    Courtesy of BMW OF NORTH AMERICA, INC.
Heater Monitoring Of Downstream Oxygen Sensor - Heater of downstream oxygen sensor consists of 2 checks:
- Resistance Check - Internal resistance depends on ceramic temperature which is influenced by electrical heater and exhaust gas temperature. For heater monitor, resistance of ceramics is measured and compared to a reference depending on heater power and exhaust gas temperature.
- Power Stage Diagnosis - During power stage on and off, control signal (input) of power stage is compared to its output signal. Also, in switched on condition, heater current is checked against a minimum limit. With these checks an open circuit as well as a short to either ground or battery voltage can be detected. See Figure.
Diagnosis Of Downstream Oxygen Sensor - Diagnostic function detects all electrical connection faults of heated exhaust gas oxygen sensor downstream catalyst, except for heater faults. A wiring interruption or a damaged sensor heating is assumed, if sensor voltage remains within a certain voltage range for a period of time. A short circuit of sensor signal wire to battery voltage is assumed if voltage of evaluation circuit is permanently above a calibration value for a period of time. A wire-to-wire short circuit between sensor signal and ground lead is assumed if evaluation voltage remains under a calibration threshold for a period of time while oxygen sensor is cold after engine start. See Figure.
Aging Monitoring Of Downstream Lambda Sensor:
- Activity Check In Normal Operation - Function checks whether sensor output voltage of downstream sensor remains permanently above or below a specified threshold. If rear closed loop lambda control which uses the sensor signal of the rear lambda sensor is active, sensor voltage must cross a threshold at least once within a specified period of time.
- Signal Performance In Fuel Cut-Off Operation - Function checks whether output voltage reaches a value below a specified lean threshold during a defined period of fuel cut-off and reasonable exhaust gas temperature. A fault is set if signal remains above threshold.