The manually-controlled air conditioning in my 2003 Ford Taurus can’t take the heat (literally). If the outside air temperature is under 85 degrees, it works like a champ, but as the day gets hotter, the cold air disappears. This happens regardless of whether I’ve driven the car previously that day. My mechanic checked the coolant level, compressor, and sensors and all were fine and the AC was cold. He left the car sit outside in the sun for a few hours and voila - no AC! (BTW, it can happen when the car is parked in my garage too if it’s a hot day). He says something is triggering a false temperature message telling the system to shut down. He wants to dig into the wiring in the system to look for a problem. This sounds like a lot of labor $$$. Any suggestions that might help him save some time (and save me some money?)
You say he checked the coolant level??? Did he check the freon? It sounds like the freon is low and as the outdoor temperature rises it just doesn’t have enough to cool the car. I was having the same problem with one of my cars earlier in the summer, I added 1 can of freon now the a/c works fine. Unless you have a set of gauges I’d recommend taking it to someone that does and have them check the low and high side pressure.
This does actually sound like a classic instance of low refrigerant. I assume that by “coolant” you meant the refrigerant level was checked - but is your mechanic an A/C system specialist?