Honeywell digital thermostat went blank, ac unit wont come on, its an 8400c, no breakers tripped

You have lost 24 volts to the tstat. It doesn't need batteries, take them out. When you g

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Do you have low voltage to the tstat? How about good batteries if any??

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I believe this thermostat will display -ac if it loses power from the furnace or air handler. Your thermostat will operate on low voltage(same as your door bell) and constantly powers your thermostat. I would check your furnace breaker, and fuses to see if there is power to the equipment

Makes sure the thermostat switch is turned to A/C or Cool

Then make sure any switch on the AC unit is turned on

then check the circuit breaker box to make sure your AC breaker did not trip (or may have been turned off by a service technician last fall).

try taking off the wall do not pull any wires wait a few minutes and re-install if this does not reset then you should just buy another...........is the power on to the unit? check the breaker panel also to ensure the unit is recieving power. the thermostat gets its power from the AC unit.

Could be the batteries in the thermostat.

site shows this thermostat is a non programmable unit and has been discontinued. none of the info sheets including a pdf shows a replacement battery. this thermostat operates off line voltage only. check for a control fuse or breaker on your heating unit.

Can you saY, Batteries!

Blank screen sounds like a fried thermosat. Abrupt outages can affect certain types of thermostats. Make a jumper wire and jump across R and G. See if you get any action from anywhere. If a fan or compressor kicks in you know you have a burned out thermosat...