The garage that diagnosed "ageing cat" is a Citroen specialist. The other garage is a place I've used before but are only a standard repair place. Another website for C5s ( a Yahoo site) regularly has messages from members who use Citroen dealers and have gained no satisfaction from them in attempting to deal with the anti pollution fault. Apparently they pay a small fortune for replacement parts such as coils, plugs etc and the fault shows again before they've even got the car home. They return and the dealers are baffled and they spend even more money trying to resolve it. All this at dealer prices. I repeat that my car runs perfectly and if I can get it through an MOT with the anti pollution fault showing then I will do so. I cannot see any point in paying any garage a small fortune in diagnostic costs and what they deem to be necessary repairs if they do not solve the problem and this seems to be the case all too often. I have arrived at the conclusion that the fault showing is the fault and that nobody seems very sure how to get rid of it ie there may not be a fault to get rid of (hope that makes sense) . It has nothing to do with any petrol sold by Tesco or Morrisons, I've never used supermarket petrol and never will. In my area the best and cheapest petrol is Shell which is frequently 1p a litre cheaper than supermarkets. Ray.