Description Of The Open Loop Diagnosis

Catalyst monitoring is a sequential diagnosis made during steady state conditions. This monitoring is intrusive.

Three phases are necessary to complete the diagnosis:

Engine stabilization
Controlled stimuli - stabilization
Controlled stimuli - diagnosis

If a problem has occurred with the downstream sensor during the catalyst diagnosis, a sensor diagnosis is done.

Fig 1: Catalyst Monitoring And Phases Diagnosis Characteristic Diagram

Courtesy of BMW OF NORTH AMERICA, INC.

VLS_DOWN: Downstream O2 sensor signal

FIL_DOWN_LAM_CAT: filtered signal for DOWN_DYN_CAT (= detection criteria) integration

FIL_DOWN_DYN_CAT: high filtered DW signal for mean richness

During the 'Controlled stimuli - diagnosis phase' the downstream sensor activity is measured and corresponds to the OSC of the catalyst. If this activity is high (low OSC) the diagnosis criteria DOWN_DYN_CAT is high.

Fig 2: Downstream O2 Sensor Signal - Graph

Courtesy of BMW OF NORTH AMERICA, INC.

If one of the monitoring conditions is not met or if the mass air flow deviates too much from the value stored at the start of this test phase, the test is interrupted and the system returns to the out of diagnosis state.

Downstream sensor diagnosis phase:

If throughout the CONTROLLED STIMULI phase, repeated several times, the downstream sensor has not reacted, the A/F closed loop mode is delayed in order to test the sensor.

If the downstream sensor sends a signal indicating a rich (lean) mixture, the injection time is forced to lean (rich) until the sensor switches over or until the end of a delay. If this delay expires, the sensor is treated as failed. This may be a result of:

A leak in the exhaust line,
A damaged sensor.

Electrical failures (short circuit and open circuit of signal and heater) are detected during the COMPREHENSIVE COMPONENTS diagnostics.

If the catalyst diagnosis has completed without any problem, the downstream sensor is treated as GOOD and a sensor diagnosis is not necessary.

If monitoring conditions for the diagnosis are fulfilled, the system informs the OBD sequencer and waits for its authorization to start catalyst diagnosis. The OBD sequencer manages the priorities in case of multiple diagnosis requests (catalyst diagnosis and O2 sensor diagnosis).