Boosted Falcon

My son has an EF fairmont.
He is having trouble with the aircon.
He took it to aircon place thinking that it needed gas. They spent an hour on it and said that it was the climate control unit not turning the aircon on and it didn't need any gas.
He bought another climate control unit from e-bay and replaced the original. All lights work but aircon doesn't work.
He has done the reset on the unit.
The compressor doesn't start and the cooling fans on the radiator don't turn on like they should when you put the aircon on.
Fan inside works but no cooling.
He has checked all the fuses and swapped relays in fuse box and was wondering what else to check.
I was reading that the aircon is also controlled by the computer but not sure how to check.
Has anyone had this sort of problem and if so had did you go about fixing it?

The low pressure switch on the receiver drier can cause dramas try bridging the plug and see how you go

Is the front clutch plate still on the compressor?

They seem to have a habit of falling off at times.

Just wondering how you reset these things?