Oxygen Sensor Monitoring
GENERAL DESCRIPTIONThe response rate of the up stream 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 O.K.:
1. 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
2. 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)
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