The high and low side pressure sensors will throw a code in VCDS if faulty.
Other causes of less than satisfactory chilling are:
Air con pump - you will know if that's dead. It won't 'clunk' into action when you put the AC onto Auto and LO.
Blocked or leaking condenser - Blocked fins is very, very common on these cars, being the front most radiator in the sandwich of 3.
Receiver drier canister thing to the left of the radiator - Dunno what that does but apparently another potential cause.
Gas leaks - usually from a cracked condenser. Fluorescent dye is the easiest way to find leaks, or a sniffer probe.
Radiator fans - these should be seen spinning when the AC is on at idle.
And lastly, the AC on the MK5 really isn't all that anyway. Mine is fully operational and I have to drive for at least 30 mins with the windows closed before I get what I consider to be cold air out of it. It's never been 'icy' cold like a lot of other cars kick out.
