Catalyst Monitoring

Catalyst Monitoring

General Description
Solution chosen to fulfill OBD's requirement is based on Oxygen Storage Capacity (OSC).

During a controlled stimuli (special A/F pulses in engine steady state conditions), the downstream O2 sensor signal is analyzed to evaluate the OSC of the catalyst.

The OSC is correlated experimentally with the global HC efficiency and HC emission during cycle.
It represents the quantity of oxygen that is really used for the oxidation-reduction reaction by the catalytic converter (stored in lean excursion and consumed in the rich excursion).







Description of the open loop diagnosis
Catalyst monitoring is a sequential diagnosis made on steady state conditions and this monitoring is intrusive.

Three phases are composing 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.







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.







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 <> state.

<> phase:

If throughout the Controlled Stimuli phase, repeated several times, the downstream sensor has not moved, 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. The following reasons may apply:
- A leak in the exhaust line,
- A damaged sensor.

Electrical failure (short circuit and open circuit of signal and heater are treated as <>.

If the catalyst diagnosis has been done without any problem, the downstream sensor is treated as <> and a sensor diagnosis is not required. 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).