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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8511
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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My AC stopped working and my Honeywell T8775 thermostat (no

Customer Question

My AC stopped working and my Honeywell T8775 thermostat (no batteries) has a blank digital display.
Submitted: 2 years ago.
Category: HVAC
Expert:  Phil replied 2 years ago.


The thermostat wires terminate inside the furnace. If you can locate those and use a multi meter to check for voltage between the red wire and the gas valve body.. and the read is less than 11 volts, then the 24volt power transformer in the furnace has failed, or the 3 amp fuse usually found on the printed circuit board has blown... if you get 24v between red and any of the other wires attached to the thermostat terminal strip in the furnace... then the thermostat is bad, or there is a broken wire between the terminal strip and thermostat.

I will coach you on any of that as needed, meantime tell me a lot about your skill sets and if you have a multi meter or not... that way I can coach you most effectively.

If I miss you tonight, I will be back first thing in the morning.


We are on the honor system here. Please remember to rate my service before you leave, the company only pays me if you rate my work. It is how I earn a living, and does not increase your charges.

If you need anything else, just let me know. You can continue asking follow-up questions at no additional cost.