P0420-P0421

CATALYST MONITOR

Monitoring Procedure
The catalyst efficiency is monitored in two ways during steady speed driving. Firstly the average value of downstream HO2S sensor amplitude is monitored. If this value is less than a predetermined limit then the catalyst efficiency is judged to be normal. If the normal judgement is not made, the outputs from upstream and downstream HO2S sensors are compared. If the outputs do not correctly match those expected from an efficient catalyst, and the overall conversion of the system falls below 60% then the catalyst efficiency failure judgement is made. The trouble code is stored when three successive failure judgements have been made. These can be, but need not be on the same trip.