As Karl says, press and hold 2 buttons at exactly the same time with engine running.
A/C button and fresh air intake button, if there is a code it will show on the small led screen that shows modes.
My 2006 didnt show a code, even though freshly regassed, but there is another check on a 3 pin connector under the bonnet that by carefully touching 2 pins with a wire/fly lead, that you can see/hear if the mag clutch is engaging on the compressor when switched on.
I person under bonnet with connector apart and lead touched to pins, to see/hear, and another inside to push the switch on cue - all with engine running and be careful of fan blades of course.
Thanks to Karl for finding that info for me, although in my case the clutch engaged momentarily then dropped out - electrical sensor.
Does the green light in the a/c switch flash when you switch on ?
It's a bas***d job to get at/change the compressor which I shied away from, my local garage owner, who I know very well said 4 hours.