Test Group XBMXV02.8LEV
General Description - Both oxygen sensors upstream from the catalyst are separately monitored for rich and lean voltage and response time (time is exposed to rich and lean periods). See Fig 1.
Rich/Lean Periods - To determine switching time, lean and rich period times are added during a fixed number of lambda controller cycles. A malfunction is registered if one or both of the times exceed thresholds which depend on engine speed and load. See Fig 2.
Oxygen Sensor Monitoring (Downstream Sensor) - Lean sensor voltage is used to diagnose sensor activity. Therefore, this check is performed during a deceleration mode (for example: engine operation mode +/- fuel cut-off). Diagnosis starts after a calculated air mass (integral) is reached at transient from any operation mode to fuel cut-off mode. Sensor voltage has to drop below a predetermined value, otherwise a fault is detected and a code is stored.
Oxygen Sensor Heater Monitoring - 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 both sensor heater current (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.
Oxygen Sensor Circuit Monitoring - Monitoring of electrical faults of sensors upstream and downstream of catalyst:
- Unplausible Voltages - Voltages exceeding maximum threshold (VMAX) are caused by a short circuit to battery voltage. Voltages falling below minimum threshold (VMIN) are caused by a short circuit of sensor signal or sensor ground to ECM ground.
- Unplausible Course Of Sensor Voltage - An open circuit of sensor upstream catalyst can be detected if voltage is remaining in a specified range after sensor has been heated. See Fig 4.