Audi Repair: Air Con ECON question, engine coolant temperature, temperature sensors

Question
QUESTION: Hello, i have just bought a 4 year old Audi A3 and can't seem to get the Air Con working. If i press the Auto Button then the ECON button also comes on. This also happens when i press the temperature button. The thing is that the ECON light or Switch won't turn off when i press the button. Any idea what may be wrong?
Many thanks
John

ANSWER: The Econ button lights when the compressor is not being switched on.  This can either be a problem with the compressor circuit (loow refrigerant perhaps, or perhaps the interior fan is not working.  It may be that the fuse in the AC circuit is blown.  Check the fuses and let me know what you find, also check if you use a defrost setting if you can hear the AC compressor switch on under the hood.  Let me known,  Jan

---------- FOLLOW-UP ----------

QUESTION: Hi ya. Thanks for giving me some pointers.
I have checked the fuses and they are all fine.
I have found out something else though which may help.
The ECON light does not come on when the inition is on but the engine is not and i do get some cold air blowing through. As soon as i turn the engine on however, the ECON lights comes on and i am unable to turn it off again.
Does this give you any other thoughts as to what could be wrong?
Thanks very much for your help
John

Answer
That is interesting, since for the air conditioning to work, the compressor has to spin, and this does not spin when the engine is off.  However this could point towards the problem.  The problem could be one of the engine coolant temperature sensors, or the A/C clutch relay is not fucntional, there is obviously an interlock which is stopping the AC compressor from being switched on.  Check the AC relay and see if it is getting the required inputs.  Most of the relays are located under the dash, you can get to them by removing the lower dash panel.  Let me know what you find,  Jan