Oxygen Sensor Monitoring

OXYGEN SENSOR MONITORING

As part of the OBD II requirements all oxygen sensors must be monitored separately for electrical integrity, heater operation, switching time and response time. In order for the oxygen sensor to be effectively monitored, the system must be in closed loop operation.

Closed loop operation is a monitoring stage whereby all emission relevant components are operating correctly and the composition of the fuel/air mixture supplied to the engine is continuously maintained within an optimum range. In order to maintain an "Optimum Range" the exhaust gas is measured and the injected fuel quantity (ms time) of the injectors is adjusted accordingly.

As An Example: 02 sensor signal indicates a rich condition the engine control module will reduce the injection time in order to reduce the amount of fuel being supplied.

Within the BMW emissions monitoring system two different Oxygen Sensors are utilized based on engine application.




Bosch DME systems use a Bosch manufactured sensor (Voltage Generator).

Siemens MS 41.x Systems use an NGK manufactured sensor (acts as a resistor).

NOTE: If an oxygen sensor fault is "currently present" misfire detection, catalyst monitoring and adaptation control will be disabled until the oxygen sensor fault is "not currently present".

Conditions For Oxygen Sensor Monitoring:





Catalyst temperature is a DME calculated value that is a function of load/air mass and time.
Conditions For Oxygen Sensor Monitoring:

Oxygen Sensor Electrical Integrity Check:

The Monitoring of the electrical integrity of the oxygen sensor is an ongoing functional check made under normal vehicle operation which pertains to faults with either the wiring, connectors, or sensor.

Part of the monitoring function includes the following checks:

If the monitored sensor voltage exceeds the maximum threshold value the DME will interpret the signal as a short to B+.

Bosch System: Short to B+ = Rich

Siemens System: Short to B+ = Lean

If the monitored voltage of the sensor remains unchanged or within a predetermined voltage range after the sensor has been heated and the engine temperature has exceeded a predefined threshold, the DME will interpret the signal as an open. (Separate fault code set - Siemens only).

Bosch System: No change = Rich

Siemens System: No change = Lean


If after completing the next consecutive diagnostic cycle and a fault is again encountered, the "Check Engine" light will be illuminated.

Causes:

Open or short in sensor circuit/wiring.

Short to B+ or Ground





Example: Oxygen Sensor Circuit (Ms 41.1 & Ms 41.2 - Siemens M52)




Example: Oxygen Sensor Circuit (DME M5.2 - Bosch M62 & M73)





Example: O2 Sensor circuits (DME M5.2.1-Bosch M62 & M73)





Associated Fault codes: Electrical Integrity

O2 Sensor Heater Check

In order for the oxygen sensor to operate correctly the sensor element must be heated. An improperly, non operating heater will not allow the sensor signal to reach its predefined maximum and minimum thresholds which can:

Result in delayed closed loop operation causing an impact on emission levels.

Result in increased emission levels while in closed loop operation.

As part of the monitoring function for heater current and voltage the circuit is also checked for an open short to ground and short to B+ depending on the values of the current or voltage being monitored.

Bosch: The heater monitoring function measures both sensor heater current and the heater voltage in order to calculate the sensor heater resistance and power. Oxygen sensor heater current is calculated via a voltage drop over a shunt resistor, internal to the control module.

If the power of the heater is not within a specified range, a fault will be set. The next time the heater circuit is monitored and a fault is again present the "Check Engine" light will be illuminated.





NOTE: The heater function is monitored continuously while the vehicle is in closed loop operation, as long as the heater is activated by the Engine Control Module.

Siemens:Pre-Catalyst Heater:

If the heater output is too low, the signal amplitude of the 02 sensor will be reduced. If the predetermine minimum low (rich) voltage and high (lean) voltage can not be obtained within a predefined time period a fault will be set. The next time the heater circuit is monitored and a fault is again present the "Check Engine" light will be illuminated.

NOTE: The heater function of the pre-cat sensor is monitored continuously while the vehicle is in closed loop operation, as long as the heater is activated by the Engine Control Module.

Siemens: Post-catalyst Heater

The rear Oxygen Sensor heater is evaluated by monitoring the amount of change that occurs on the rear Oxygen Sensor signal during a deceleration/fuel cut-off phase.

During a deceleration phase (refer to Federal Test Procedure) the post cat 02 sensor is switched to a load resistance value of 100 kW (normal sensor resistance @ 30 kW). By switching the resistance of the sensor to 100 kW the sensor voltage is expected to remain within a fixed range (lean). If the heater is operating correctly the 02 sensor signal will remain within a predefined voltage range.

The system monitors the number of cycles for which the sensor voltage remains within the fixed range (once per diagnostic cycle). If the length of time that the sensor remains within the fixed range is less then the predetermined limit, a fault will be set. During the next "customer Drive Cycle" if the heater circuit is monitored and a fault is again present the "Check Engine" light will be illuminated.

NOTE: The heater function is monitored once per trip while the vehicle is in closed loop operation (Bosch or Siemens System).





Example: Post catalyst Oxygen Sensor Heater Check (Siemens).

Causes:

Open or short in sensor circuit.

Improper sensor operation (resistance to high/low).

Insufficient voltage supplied to heater.




Example: Oxygen Sensor Heating Circuit (MS 41.1 & M5.2.1 - Siemens M52/E39 & Bosch M62/73)

NOTE: MS 41.1 & MS 41.2 for E36 will get power from a relay.




Example: Oxygen Sensor Heating Circuit (DME M5.2-Bosch)




Associated Fault Codes: Heater Circuit

Switch Time Monitoring:

The calculation/evaluation of switching time is made as indicated in the graph below. The transition/switching time from lean/rich and nc Mean is measured and compared with a predetermined maximum value (threshold) depending on engine speed (RPM) and air mass/Load (kg/h).

A relative range of 10-90% of the signals predefined/expected min/max. amplitude is used to determine how quickly the sensor signal changes from lean/rich and rich/lean. During the monitoring period if the signal suddenly reverses direction that particular cycle is not used.

Since switching time fluctuations can occur the measured results are averaged over the range of a predefined number of cycles, before a comparison to the maximum threshold limit is made. If the maximum threshold is exceeded a fault is stored in the DME. Whenever switching time is again monitored and the maximum threshold is exceeded the "Check Engine" light will be illuminated.

NOTE: The switch time function is monitored once per trip while the vehicle is in closed loop operation (Bosch or Siemens System).




Example: Pre Catalyst Oxygen Sensor Signals (Siemens)

t R/L = Switch Time from Rich to Lean.

t L/R = Switch Time from Lean to Rich

Causes: Aging/Deterioration Of Sensor.




Associated Fault Codes: Switch Time

Response Time Monitoring:

The response rate (Oscillation Frequency) of the 02 sensor signal is dependent on engine speed and load/air-mass. In order to determine if the 02 sensor signal is cycling/oscillating correctly the length of time a signal remains at rest in either the lean or rich condition (see graph below) is measured and compared with a predetermined maximum value (threshold), dependent on engine speed (RPM) and load/air-mass (kg/h). If the maximum "at rest" threshold is exceeded a fault is stored in the DME. Whenever the response time is again monitored and the maximum threshold is exceeded the "Check Engine" light will be illuminated.

NOTE: The response time is monitored once per trip while the vehicle is in closed loop operation (Bosch or Siemens System).




Example: Pre-catalyst Oxygen Sensor Signal (Bosch).

Causes: Aging/deterioration of sensor




Associated Fault Codes: Response Time