Hi Alf, if front and rear lambda sensors are switching in unison then the cat is not working. The ecu checks for cat efficiency when fueling is in closed loop and car is cruising at approx 50 mph. Ecu monitors rear lambda sensor, if cat fails three times then MIL is illuminated. Driving round town may not illuminate MIL, however, dual carriageway or motorway driving will. Asta lavista, it will be back. By the way you cannot reset the ecu for this fault, you can only clear the fault code, which should clear as the conditions for monitoring are no longer being met. Regards, SteveL.