There are a couple of other things to be aware of. Air conditioning systems work by compressing a refridgerant gas (R134a), condensing it to a high pressure liquid, then passing it through a valve to reduce the pressure and temperature, and finally evaporating it back to a low pressure gas. The second law of thermodynamics states the heat always seeks out cold areas, and so it's more accurate to say that the A/C system removes heat rather than produces cold. The system must have the correct amount of gas, and this information is usually stated on the front panel of the vehicle. Too much gas will mean the system will operate just as inefficiently as having too little.

There will be a switch on the high pressure line which will switch of the compressor if the pressure gets too high or stays too low. Insuficient gas is usually the problem here, but as you say that is not the problem, but the switch may have failed. Also there will be a temperature sensor on the evaporator (inside the vehicle) will switch off the system if the evaporator gets too cold. The build up of ice could damage the evaporator. The condensor (fitted in front of the radiator) has a cooling fan which must be working for the A/C system to work. The fan will be switched on by a relay. Check the relay and fuses for 12v feed.

I don't know specifically about Citroen, but I have VCDS for VW/Audi/Skoda/Seat. This will check the Climatronic system but not the more basic system.