While running my upstairs A/c the thermostat stopped working and went blank. The tech replaced a fuse in the attic furnace and also the thermostat in my bedroom. The system began to work. As soon as I turned on the A/c two weeks later the thermostate went blank and it sounded like the fuse popped again.

Call back the people that fixed it. They should stand behind there previous work at least for 30 days.