On my 2000 S-Type 4.0 I have been having problems with the A/C. It will blow cold for a period of time and then the air suddenly gets warm. I will then park the vehicle and restart it and the A/C is fine again. There doesn't seem to be any time period in which this occurs but it's been going on for a couple of weeks now. No error codes are coming up and the other night, I let it run for almost an hour in my garage and it was cold. Today, I drove about 5 miles; started off cold and then blew warm until I stopped, turned off the engine and restarted it and then it was fine again. Does anyone have any clue what could be causing this?

 

Probably the DCCV (use Search & see FAQs) aka heater aka water valve.