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.

Gilly