air con won't cut in

[Bloomer]

an anyone help with an air con fault? I have got a 2004 Leon Cupra TDI and the air con won't cut in. The error code is showing that the pressure switch has an earth problem but I have changed the switch twice to no avail. Ive also changed the other switch under the rad and changed the control unit but it still won't work . I have had the wiring checked by an electrical engineer and he can't find a wiring fault and the system is fully gassed. The fans won't cut in or the air con clutch but when you send a direct live and earth to the cluth it kicks in straight away with no problem.
Also should the fans work all he time or just when the car gets hot?

 

[hopper]

ave you checked the fuse box on top of the battery, normal problem is these melt

 

[Bloomer]

I haven't checked the fuse box but it has been to an electrical specialist so I would have thought he would have done. I will have a look at that later. Thanks for the tip.