Oxygen Sensor Monitoring

GENERAL DESCRIPTION





Both oxygen sensors upstream from the catalyst are separately monitored for rich and lean voltage and response time (time is is exposed to rich and lean periods).

MONITORING PROCEDURE

1. Rich/lean periods

For determine 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.





2. Flow Chart of Monitoring Oxygen Sensor Switching Time (lean to rich)

3. Oxygen-sensor monitoring (Downstream sensor)

The lean sensor voltage is used to the diagnosis of the sensors activity. Therefore this check is performed during a deceleration mode i.e. engine operation mode -> fuel cut-off.

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.

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 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