Test Group YBMXV02.8LEV

Oxygen Sensor Monitoring General Description -  Both oxygen sensors upstream from the catalyst are separately monitored for rich and lean voltage and response time (period monitoring and jump period monitoring). See Fig 1.

1. Monitoring Procedure
   
   
   • Switching Time -  For monitoring the switching time, the time from lean to rich mixture and visa versa, are determined and compared with predetermined maximum values which depend on engine RPM and air mass.

     Relative thresholds (10-90% of min./max. amplitude) are used to determine the oxygen sensor switching time from lean to rich and visa versa.

     The sensor signals are monitored for reversal points while the switching times are being measured. If a reversal point is detected this oxygen sensor signal is not used to determine the switching time. Due to switching time fluctuations the results are averaged during the monitoring cycle (115 lambda control cycles) before comparing with the thresholds.

   • Rich/Lean Periods -  For determining the switching time the 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(s) the thresholds which depend on engine speed and load. See Fig 2.
2. Oxygen Sensor Heater Monitoring -  For proper function of the oxygen sensor, the sensor element must be heated. A non functioning heater delays sensor readiness for closed loop control operation and influences emissions. Disconnection, short-to-ground and short-to-battery monitoring and oxygen sensor heater current and voltage monitoring are performed in the standard diagnosis function.
   • Heater Circuit Oxygen Sensor Upstream -  Due to slow sensor heating, in the case of low oxygen heating output, the rich voltage does not reach its maximum value within a defined time. This reduces the oxygen sensor signal amplitude which permits oxygen sensor heating output to be monitored. The malfunction detection is activated every 240 ms independent of engine operation.
   • Oxygen Sensor Monitoring (Downstream Sensor) -  Lean sensor voltage is evaluated for diagnosis of the heater circuit. Therefore this check is performed during deceleration/fuel cut-off phases. The diagnosis starts after a calculated air mass (integral) is reached at transient from any operation mode to the fuel cut-off mode. The sensor voltage has to drop below a predetermined value, otherwise a fault is detected and a code is stored.
3. Oxygen Sensor Heater Monitoring -  For proper function of the oxygen sensor, the sensor element must be heated. A non functioning heater delays the sensor readiness for closed loop control and influences emissions. The monitoring function measures both, sensor heater current (voltage drop over a shunt) and the heater voltage (heater supply voltage) to calculate sensor heater resistance. The monitoring function is activated once per trip, once the heater has been switched on for a certain time period and the current has stabilized. For flow chart of monitoring function of oxygen sensor heating, see Fig 3.
4. Oxygen Sensor Circuit Monitoring -  Monitoring of electrical faults of sensors upstream and downstream of catalyst, are as follows:

   • Unplausible Voltages
     
     
     1. ADC voltages exceeding the maximum threshold VMAX are caused by a short circuit to battery voltage.
     2. ADC voltages falling below the 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 the sensor upstream catalyst can be detected, if the ADC voltage remains in a specified range after the sensor has been heated.

Fig 1: Oxygen Sensor Switching & Response Time Chart

Courtesy of BMW OF NORTH AMERICA, INC.

Fig 2: Oxygen Sensor Switching Time Flow Chart

Courtesy of BMW OF NORTH AMERICA, INC.

Fig 3: Oxygen Sensor Monitoring Function Flow Chart

Courtesy of BMW OF NORTH AMERICA, INC.