Audi A6 climate control not cooling

Hi,

I recently purchased a '02 Audi A6 1.9 tdi (96k miles). Everything seemed fine until spring kicked in and the weather became warmer. The climate control does not cool at all. The vents produce air but it is not cold. In fact, pressing ECON (a/c off) gives better results although far from cold air.

I have:

(1) Tried VAG-COM but had too many communication errors to get any fault codes.
(2) Read on the internet how to access the diagnostics on the climate control itself but because it is a dual display, I have not been able to get a definitive list of fault codes to figure out what is wrong.
(3) Had the refrigerant levels checked and they are fine.
(4) no problem communicating with all other control modules in the car using VAG COM without a problem, so I suspected that the air con controller was faulty (previously had heard buzzing sound from unit). I got a replacement climate control display unit and installed but still the problem remains.
(5) I checked all fuses and relays and they seem OK.
(6) When I switch on ignition (without starting engine) and just before the aircon comes on, I can hear a faint clicking behind the dash which seems to be coming from deep inside the dashboard. Could this be the air flap motors malfunctioning?
(7) When the car is running and I check under the bonnet, the refrigerant pipes are not cold at all. Would malfunctioning air flaps cause this?
(8) I ran VAG COM on the replacement unit and still communication errors but repeated my attempts until I got a scan of the unit and found one fault:

Air Quality Sensor - Electric Circuit Failure

I checked the sensor (located in plenum chamber next to pollen filter) and the mount for the sensor was broken (no idea how this happened) with the sensor lying on the bottom of plenum chamber (all connections seemed intact). I have ordered a replacement sensor and waiting on arrival but I would like your opinion on this - I cannot see how this sensor circuit failure could be preventing cold air from circulating (air con seems fine otherwise). Am I wrong? I thought that the problem might be likely due to a faulty pressure switch but VAG COM did not give any other faults.

I would appreciate any advice at all. I am desparately in need of cold air!