Update:
I have appended a section on fan troubleshooting. It is actually for a Mk VI, but the steps for the Mk V are very similar, but the names in the measuring value blocks may differ slightly.
The A5's come with either Climatronic or Climatic (replaces manual, and is a simplified Climatronic.)
These compressors do NOT have an electric clutch, but rather a coupling with a break-a-way feature that is supposed to fail when the compressor locks up, so your serpentine belt does not break (and perhaps ruin your timing belt.) Make sure the center shaft of your compressor is turning before doing any other troubleshooting.
The compressors have an electric valve (N280) Refrigerant Control Valve (RCV) which controls the compressor's variable displacement based on evaporator temperature. This is as opposed to the mechanical valve inside an earlier (A4) compressor. Of course there are what appear to be dozens of interlocks and control override features to complicate the picture - but they can be checked with VCDS.
The N280 RCV is prone to sticking. VW says the only fix is to replace the compressor, but many people have been able to replace them easily and successfully . The symptoms of this is greatly delayed and erratic cooling from your AC system with all other things being healthy, or simply no cooling.
The fans on the A5 are unfortunately no more reliable than the A4 fans have proven to be.
The fans on the old mk IV manual AC system should start in slow as soon as the AC was asked to turn on, it did not matter if the clutch engaged or not.
The fans on the MkV systems must see AC pressure rise to over about 140 psi before they come on in the AC mode.
Luckily there are very good troubleshooting aides for the Mk V system built into its controllers - a VCDS is invaluable.
Fans - Go into the Engine Controller, OUTPUT TESTS, and choose the one for fan testing circuit one (on BRM engines you must choose sequential tests, and start with the engine running, then go through the test sequence until you reach the fan test - note - expect your engine to die at the ASV closure test; do not restart it, just keep going through the test sequence). This should cycle BOTH fans from off or very low speed to full speed in a periodic cycle.
The fan controller is built into the large fan on these cars. It develops the pulsed power signal that drives both the large fan and the small fan; because of this both fans should always turn at the same speed.
There is another test in this module for the AC system, but it is of minimal value. It calls itself AC clutch relay control circuit test, but don't be fooled - your compressor has no clutch. It does test that the ECU can tell your compressor to unload. If you run this test, do so on a warmed car, and watch for the changes in fuel as the compressor is allowed to load and unload (on some models you can see the pressure cycle as well). Have the AC on, engine on and warmed, AC demanded full cool.
Then go to the AUTO HVAC controller ADV Measuring values:
Choose
refrigerant pressure
Air-Conditioning Operation information
Compressor Shut-off Requirement
compressor current, specified value
Temperature after evaporator
Look at these closely.
If you have .825 amps or so requested current that is max. The refrigerant pressure should be high, and the temperature after the evaporator should be low - 2 to 4 C or so.
A bar of pressure is roughly 15 psig, so you can do the math ( or go to a units conversion site on line and let it do the math for you) to see if your high pressure sender agrees with your mechanical gauge set.
If anyone figures out the details on the basic settings tests and the Output tests for the AUTO HVAC module, please post them in this thread.
The compressor shut-off requirement is the status of the interlocks, and can tell you what may be preventing operation. As long as it says "no shut-off signal is present" and your current is high, then the pressure should be high and the temperature after the evaporator is low. If the pressure is low (and you have a good charge in the system) then the RCV is probably bad. Do verify that the compressor is turning, which it must be unless the break-away protective system has broken free.
The most common failure on these cars AC system is the RCV, Refrigerant Control Valve built into the compressor.
Next is probably the fans.
Then the high pressure sensor.
AC Compressor Refrigerant Control Valve Make sure you get the proper valve for your compressor.
Some AC shut-off codes:
A/C cut-off Codes
0 = Compressor on (no shut-off requirement of the A/C compressor detected)
1 = Compressor off (refrigerant pressure was or is too high)
2 = Compressor off (blower motor circuit)
3 = Compressor off (refrigerant pressure was or is too low)
4 = Compressor off (display not currently intended)
5 = Compressor off (no engine start or no engine speed detected)
6 = Compressor off (A/C system switched off, ECON-mode activated)
7 = Compressor off (A/C system switched off, blower switch in position 0)
8 = Compressor off (outside temperature was less than 1.5°C and is still less than 2.5 °C)
9 = Compressor off (display not currently intended)
10 = Compressor off (Low voltage)
11 = Compressor off (display not currently intended)
12 = Compressor off (shut-off request from Engine Control Module (ECM) via CAN)
13 = Compressor off (voltage supply terminal 30 too high)
14 = Compressor off (Evaporator Temperature)
15 = Compressor off (display not currently intended)
16 = Compressor off (activation A/C Compressor Regulator Valve -N280- implausible)
17 = Compressor off (no signal or implausible signal from High Pressure Sensor -G65-)
18 = Compressor off (engine speed too high at standstill)
19 = Compressor off (shutoff request from Vehicle Electrical System Control Module via CAN)
20 = Compressor off (refrigerant loss-currently V8 Touareg only)
21 = Compressor off (Outside Temp < 2 degrees C, without recirc mode on)
22 = Compressor off (Outside Temp < 2 degrees C, Inside Temp. < 10 degrees C)
