2002 Cadillac Deville Catalylic Converter

It is possible but not likely that both catalytic converters will fail at the same time. I would recommend only replacing the failed cat. However you can check the other cat simple by hitting it with a rubber hammer. If you hear any rattling inside the cat it has begun to fail by seperating internally. (This is one method of checking cats) As far as aftermarket and factory cats go, I always recommend using factory parts as they tend to have higher standards for manufacturing than aftermarket manufactures. However the cost of catalytic converters is very high and at the dealership you will always pay more. Also ask about any warranties on the new cat as this may help you make your decision.