Oxygen Sensor Monitoring

Monitoring of Upstream Oxygen Sensors

General Description of Upstream Oxygen Sensor Monitoring





Both oxygen sensors upstream from the catalyst are separately monitored for rich and lean voltage and response time (period monitoring and jump period monitoring).





Monitoring Structure of Upstream Oxygen Sensor

Monitoring procedure of Upstream Oxygen Sensor

Overall period time monitoring of Upstream Oxygen Sensor





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 depends on engine speed and load.





Jump period monitoring of the Upstream Oxygen Sensor

Monitoring Procedure of Downstream Oxygen Sensors

Monitoring from rich to lean intake mixture

The lean sensor voltage is used to the diagnosis of the sensors activity. Therefore this check is performed during a deceleration 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 and a defined time in deceleration fuel cut-off. The sensor voltage has to drop below a predetermined value otherwise a fault is detected and a code is stored.

Monitoring from lean to rich intake mixture

When the diagnosis conditions at deceleration fuel cut-off are not fulfilled, the diagnosis is carried out in opposite direction of the oxygen sensor voltage.

For a positive diagnosis result the signal must overrun a threshold after deceleration fuel cut-off. To ensure the diagnosis the mixture can be short-term enriched independent of the respective operating conditions.

Oxygen Sensor Heater Monitoring

General Description of 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 continuously both, the sensor heater current as well as the heater voltage (heater supply voltage) to calculate the sensor heater resistance.





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 Oxygen Sensor Heater Monitoring

Oxygen Sensor Circuit Monitoring

Monitoring of electrical faults of sensors upstream and downstream of catalyst

Not plausible 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

Not plausible 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