Oxygen Sensor Monitoring
Oxygen Sensor Monitoring for Test Groups:2BMXV04.4LEV: Models: 540i, 540 sport wagon
2BMXT04.4E53: X5 4.4
2BMXT04.6XHP: X5 4.6is
General Description
The response rate of the upstream oxygen sensor is monitored by measuring the period of the lambda control oscillations.
Monitoring Structure
Flow Chart of Monitoring Function
Diagnosis Procedure of the Monitor Sensor (Downstream)
The activity of the monitor sensor after reaching operating conditions, is determined by two different procedures.
A) Oscillation Check (Line Crossing)
If the following checks are correct, the monitor sensor will be regarded as O.K.:
- The monitor sensor signal (sensor voltage) is > than the nominal value of the TV- Correction and voltage increases, if the lambda - control goes to the lean side, or
- The monitor sensor signal (sensor voltage) is < than the nominal value of the TV- Correction and voltage decreases, if the lambda - control goes to the rich side.
B) Fuel Cut-Off Check
Additionally to the above mentioned checks the signal behavior of the monitor sensor is checked in case of fuel cut-off.
Therefore the monitor sensor voltage has to be below a given nominal value in case of fuel cut-off.
If the monitor sensor is detected defect with the checks A or B, a fault code is stored and the MIL is illuminated at the next driving cycle.
Oxygen Sensor Heater Monitoring (Up-and Downstream)
General description
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 the sensor heater resistance.
The monitoring function is activated once per trip, if the heater has been switched on for a certain time period and the current has stabilized.
Circuitry for heater controlling and heater current measuring
Characteristics:
- ECM - controlled switching on of the sensor heater
- one shunt for each sensor heater upstream and downstream of catalyst for current measurement
Flow Chart of Monitoring Function
Oxygen Sensor Circuit Monitoring
Monitoring of electrical faults of sensors upstream and downstream of catalyst
Unplausible voltages
- ADC - voltages exceeding the maximum threshold VMAX are caused by a short circuit to VBatt
- 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 is remaining in a specified range after the sensor has been heated
Oxygen Sensor Monitoring for Test Group 2BMXV04.4LEV Models: 745i, 745Li
Heater Diagnosis of Upstream Oxygen Sensor (UEGO)
General Description
The Universal Exhaust Gas Oxygen sensor (UEGO) is ready for operation at temperatures above 600°C. In most cases exhaust gas temperature is not sufficient for heating, and so the sensor heating is needed for a proper functioning of the sensor.
The diagnosis consists of three checks:
Operation readiness
Sensor readiness depends on heater performance. That's why time delay between "heater on" and "operation readiness" is monitored. Therefore sensor readiness is checked at a calibrated time after the 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, the control signal (input) of the 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.
Monitoring Overview
Diagnosis of heater performance
Diagnosis of Upstream Oxygen Sensor (UEGO)
General Description
Figure 1 shows the output voltage characteristic of the universal oxygen sensor (UEGO).
The diagnosis of upstream oxygen sensor consists of four checks, of which some are subdivided into several checks.
Offset check
The offset check monitors incorrect lambda measurement due to shunting effects. If the lambda- offset of downstream-control exceeds a threshold, a fault code is set.
Heater coupling check
The heater coupling check monitors low impedance coupling between heater and sensor, which can cause lambda modulations with heater pulse rate. If the difference of consecutive lambda values exceeds calibration, a fault code is set.
Dynamics checks (slow response)
Due to ageing, sensor dynamics can decrease.
The dynamics check in normal operation mode compares measured and estimated (modelbased) lambda-behavior, caused by artificial lambda modulation. If the ratio of measured and estimated amplitude is below calibration, a fault code is set.
Plausibility check
a) UEGO is not active:
If lambda value is close to 1.0 for a period of time, a fault code is set. Monitoring conditions for plausibility check a):
9. downstream oxygen sensor voltage (HEGO) indicates lean or rich mixture
b) Sensor current high (lambda-LSU lean): If lambda value exceeds calibration, but downstream oxygen sensor (HEGO) indicates rich mixture, a fault code is set.
a) Sensor current high (lambda-LSU rich): If lambda value is below calibration, but downstream oxygen sensor (HEGO) indicates lean mixture, a fault code is set.
Monitoring Overview
Heater Monitoring of Downstream Oxygen Sensor (HEGO)
General Description
The heater of downstream Nernst Type 02 sensor consists of two checks
Resistance check
The internal resistance Ri depends on the ceramic temperature which is influenced by the electrical heater and the exhaust gas temperature.
For the heater monitor the resistance of the 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, the control signal (input) of the power stage is compared to its output signal. Also in switched on condition the heater current is checked against a minimum limit. With these checks a wire interruption as well as a short to either ground or battery voltage can be detected.
Monitoring Overview
Diagnosis of Downstream Oxygen Sensor (HEGO) General Description
The diagnostic function detects all electrical connection faults of the heated exhaust gas oxygen sensor downstream catalyst, except for heater faults.
A wiring interruption or a damaged sensor heating is assumed, if the sensor voltage remains within a certain voltage range for a period of time.
A short circuit of the sensor signal wire to battery voltage is assumed, if the voltage of the 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 the evaluation voltage remains under a calibration threshold for a period of time, while the oxygen sensor is cold after engine start.
Aging Monitoring of Downstream Lambda Sensor
General Description
Activity check in normal operation:
The function checks whether the sensor output voltage of the downstream lambda sensor (Nernst type) remains permanently above or below a specified threshold.
If the rear closed loop lambda control which uses the sensor signal of the rear lambda sensor is active, the lambda sensor voltage must cross a threshold at least once within a specified period of time.
Signal performance in fuel cut-off operation:
The function checks whether the 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 the signal remains above the threshold.