Help 480 poor running

Hi I need some advice please, my 480 es 1.7 started to run poorly after a long trip, before which I used some injector cleaner addative in the petrol. Now it starts well warms up slightly, then runs as if its short on petrol. I thought it might be dirt in the fuel injectos system that might have been dislodged by the long trip and the injector cleaner, but somebody told me that it could be the catalytic converter, which may have failed and blocked the exhaust pipe. The car is slowly getting worse and worse. Any ideas? Can they run without the catalytic converter? Im thinking of selling it anyway so dont really want to spend £400 on a new cat, for a car that I hope to get £1000 for!
Cheers Tim