Oxygen Sensor Monitoring

GENERAL DESCRIPTION





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

MONITORING PROCEDURE

1. Switching Time

For monitoring the switching time the time from lean to rich mixture and vis versa are determined and compared with predetermined max. values which depend on engine rpm and air mass.

Relative thresholds (10 - 90% of min./max. amplitude) are used to determine the oxygen sensor switching time from lean to rich and vis versa.





The sensor signals are monitored for reversal points while the switching time are being measured. If a reversal point is detected this oxygen sensor signal is not used to determine the switching time.

Due to switching time fluctuations the results are averaged during the monitoring cycle (115 lambda control cycles) before comparing with the thresholds.

2. Rich/lean periods

For determine the switching time the lean and rich period times are added during 115 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.





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

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.

Disconnection, short-to-ground and short-to-battery monitoring and the oxygen sensor heater current and voltage monitoring are performed in the standard diagnosis function.

Heater circuit - oxygen sensor upstream

Due to slow sensor heating in the case of low oxygen heating output, the rich voltage does not reach its maximum value within a defined time. This reduces the oxygen sensor signal amplitude which permits the oxygen sensor heating output to be monitored.

The malfunction detection is activated every 240 ms independent of the engine operation.

Heating output - oxygen sensor downstream

The lean sensor voltage is evaluated to diagnosis the heater circuit. Therefore this check is performed during deceleration/fuel cut-off phases.

The sensor downstream must be switched to a load resistance of 100 k-ohms (normal operation 30 k-ohms).

During this check the oxygen sensor signal output voltage is controlled to a fixed value (lean limit). If the number of cycles during which the sensor signal lies within the defined limits is smaller than the limit, a fault will be stored.

The sensor signal is checked every 20 ms during the heater circuit diagnosis.