I have a carrier heat pump. Left to run errands, working fine, came home and the thermostat was blank. Power is fine, breakers are on. Was able to put batteries in the unit (never knew I could do that) and get a read out, change functions etc. But it doesn't "talk to" the heat pump - so no air blows through the vents. Inside flap has this information: 960-030280-10 REV A SAE ABS