Well if the computer has decided it's a "fault" i'm fairly certain it will need to be turned off or you may well have knackered the sensor by taking it off and on etc. Try to find someone with a reader who can turn it off.
me, although it a crap one - will only say o2 fault, although it'll clear it down for you and if it comes back you'll be able to tell if there's an issue or not.
How long have you had it on since refitting the cat ? Mine comes on for a few days at a time, but will turn itself off. It seems as though it's quick to come on with the slightest spurious reading, but a lot slower to realise that everything is fine.