It was probably a P0420 code which compares the pre and post cat hego sensors , if the post one still shows big changes in oxygen content it throws up the code as it assumes the cat is no longer working , but I have had this code come on for no reason , it may come back on as it could be on a warm up cycle and it did not get up to temp so was not very efficient and the light came on , but after several drive cycles it will self extinguish as it has not seen the same fault on those cycles .