4.1.1 Description

The Catalyst monitor operates once per trip. It waits until all entry conditions are met.

Once all the entry conditions are met, the monitor will start to run. The fuelling is cycled rich and lean by approximately ±3% to achieve a reaction at the downstream oxygen sensor: this process is called dither. At the start of any monitoring period, a short delay will occur before data collection in order to ensure that the fuelling is stable when the diagnosis takes place. If for any reason, the entry conditions become invalid, but the monitor has not yet completed then the result and execution time data are retained. If the entry conditions are again fulfilled, the monitor will resume with the stored data, unless there have been more than four attempts to run the check, in which case the monitor will clear the accumulated data and restart the diagnosis.

After the monitor has run for a sufficient period of time, the results are calculated. These are determined by accumulating the locus of the downstream oxygen sensor signal against the accumulation of the upstream oxygen sensor, i.e. the more active the downstream sensor, the less oxygen storage capacity the catalyst has, resulting in a correspondingly higher locus value. With a correctly operating catalyst, the downstream sensor is not so active, so lower locus values are obtained than would be recorded with a faulty system.

If the accumulated count is lower than a calibratable threshold then the catalyst diagnostic test has been passed. If the accumulated count equals or exceeds the calibratable threshold then the catalyst system has a problem and the appropriate DTC will be stored.