My dash display shows no temperature and the ac just stopped working, why

I see it has been a while back that you were having this problem, but I am having the same problem. It's been going on for a very long time. I have discovered that it happens if I'm using the heat. Had it fixed once but they couldn't determine if it was the thermostat or a sensor. (they replaced both) . It has been fine until I started using the heat again. I would appreciate any input that you could provide. Thanks