I agree with the poster on 7/15/03, I think it's low on freon. AC systems on these cars are just always full of DTC's which don't mean a damn thing--false code city in other words.
I'd get it to a shop specializing in either MB or AC systems and get the fill level checked and refilled if neccesary and a leak check done. Thankfully the 202 isn't famous for evaps leaking
The hissing noise is probably just the freon spraying through the expansion valve, they'll sometimes do this under certain weather (heat) and fill levels.