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I have had this issue the past 2 summers, and it's time to finally figure it out and get the issue resolved.

On long road trips with the A/C running on "Cool" or close to it (16.0 to 18.5 degrees C), the system works well for about 45mins. Duct temps with a independent temperature probe show about 2 degrees C at the dash vent above the cup holder. But then the whole system starts to blow warmer and warmer air, minute by minute. If left alone, the vent temps reach 30 degrees C. If I select "ECON" mode, the duct temp takes a few minutes to start to drop, and then after 10 mins it's back down to about 10 degrees C, so I switch ECON off and the system drops the duct temp back towards 2 degrees C again, but this time it only takes about 15mins until it starts warming up again. I have to repeat this process over and over and over.

Outside temp's are about 25 degrees C to 35 degrees C, and this ONLY happens when it's warm/humid outside. In winter the car will chug out 2 degree air all day for hours on end. My VAG Scanner doesn't show any faults, so I'm stumped as to where to start.

My initial thoughts were that the Evaporator may be freezing over? Or I have a dodgy sensor somewhere in the system. I've had the system checked at an air conditioning workshop, but he couldn't fault it.

Has anyone experienced this/got any tips to try work it out?

THANKS LEGENDS!!

Do you have any water running out of the air con drains? Can you get under the car to check the drains are clear of debris?
It does sound like it's freezing up. Wonder if the thermostat is faulty in the heater box?

Yeah, there is regularly pools of water under the car after I've shutdown. I feel like possibly the evaporator is freezing over. One time after using ECON for 5 or so mins I feel like perhaps some ice flew out the vents. But I can't be sure.

When you turn it to econ it turns the air con off. If you leave the engine running with the air con and all the windows shut, see if you can hear the compressor kicking in and out. It sounds like a thermostat issue to me and the coil is freezing up then defrosting when you turn it to econ.

My suggestion would be to go to place that does aircon and is friendly, run the system with the pipes hooked up to the aircon machine gauges. clearly it runs ok but struggles in summer could be a pressure problem
Watch what the gauges are doing, it'll highlight any system issues like a blockage.
for reference the low side when running will drop from static of around 5-6bar to 2bar and the high side should be static of 5-6bar and running around 10bar. have a google on what faults cause what pressures outside of that range.

There is the silly filter expansion valve somewhere in the system which can get clogged, will have to drain the system before removing it for checking/cleaning.

on a different car but i have seen high pressure side issues caused by a faulty drier unit (blocked)

Also there is coding on the heater control unit which i advise you check every adaption channel and see if theyre correct. i do have the audi technical document stating which to use for different versions of cooling..... japan is the best if you want to freeze your nipples off

BurgerBarley - did you get this issue resolved? I seem to have the same issue.

(also, I'm sure ice flew out of one of the vents)

@F4rdy Please could you kindly post up the Technical Document for the heater control unit. I'd like to have a play with mine using VCDS.