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I've got a 66 plate A4 avant. Recently the air con is showing as off when the engine is on. When I try to change it to on or eco they are greyed out and I can't select them. I can select eco or on when just the ignition is on but I'm not sure if it's actually working. After a bit of googling it either needs as regas, a new pressure switch or an new compressor. Anyone else had any similar problems??

Hi, new to the forum and this is my first post. Before I begin, I have sound mechanical knowledge, pretty much know what each air con component looks like / is and can do basic task such as replace parts if they are unbolt / bolt or unscrew / screw but I only have a basic tool kit and don’t have things such as electrical multi meters etc... I have recently bought my wife a 2003 1.8 TT which has an issue with the AC, it wasn't working on purchase. The control unit inside the car works fine, economy turns on and off and all switches / knobs / buttons seem to do what they are supposed to but no 'Air-Con' cold air produced. I've had it in to an Audi / VW specialist in the area for a re-charge, oil and UV dye. After this the garage told me it was working fine after the re-charge and all was well, but when the wife brought it home (20 minute drive) I didn't think it was any different, luke warm air at best so back it went again. On inspection under UV light this time they found a small issue / leak with the condenser dryer, so that's been replaced and the system re-gassed again. Again, they included a UV dye and before they told us the car was ready they inspected again. They told me no further leaks could be found and said it was working fine. My missus picked up the car, phoned me to say it was actually working great but by the time she had again driven home it was not blowing cold again, as if the eco had been turned on and it was just using the outside air is the best way to describe the temperature. But the Eco was not on, it was set to the lowest temp it can go to! We were out at the weekend and on occasion it kicked in and worked, ice cold 'Air Con' air blown into the cockpit, but then we stopped, get out, go shopping etc and when we return and fire up the engine it's just cool / luke warm again, outside air and not air con! I took it back to the garage, they have given the system a thorough check and found nothing wrong, they said the compressor has no issues and works, the pressure gauges on their equipment read what they should when the compressor is running and they again examined it under UV and found no leak. They also said the system pressure is the same as it was when they re-gassed it last time so it's not losing refrigerant, and sods law it was blowing cold / working when they inspected it. I have asked them if they are actually sitting in the car outside in the sun when they inspect it, they said no, it's in the workshop! for some reason I think that may have something to do with it, as if the system is cutting out when it has to work hard if that makes sense? They said their best guess at this time would be the High / Low pressure switch and have quoted nearly £200 in part, labour and re-charge!!! After reading some forums on-line I'm slightly losing faith in the garage as it seems a re-charge won’t be required, apparently there is a valve to control the release of coolant when the pressure switch is replaced, does anyone know if this is correct? My other issue with the garage after reading the forums is apparently if the pressure switch is at fault it should give out a fault code, surely the garage would have plugged it in to check for faults at the outset? if they haven't plugged it in are they simply guessing at a diagnosis? Finally, they told me the pressure switch can only be purchased from Audi, yet I can find loads on line (depending on which one I have on my car) for pittance, £20 - £30 and apparently it’s really easy and straightforward to replace? So, does anyone out there think they can shed some light or offer some advice for me, a point in the right direction would be great and if I am looking at a pressure switch can someone let me know where to locate it on my 2003 TT and maybe offer a picture of what it looks like? Any help or advice would be greatly appreciated! Andrew