I can now confirm that gas readings at idle are virtually identical to fast idle. Thanks for your response Mike, you have confirmed my thoughts. The trouble is that as I work on older cars this is the first one I have had with cat monitoring problems. I have read previous posts on this subject and due to the aftermarket cats appearing to be shrinking (saw one recently that was smaller than a baked bean can) I had already decided that I would only fit genuine on a vehicle with this type of EMS. The customer is not short of the cash but as she doesn't like the car, she may use it as an excuse to get a new one, so I may never find out. I will let her know my findings and keep you updated if I replace it. Many thanks, Bryan.