Question about Honeywell T8400c1099 Electric Heat/cool Thermostat 145793

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Honeywell T8400c Blank screen

Has been working fine till today. Noticed it was getting colder inside so went to adjust the thermostat and the screen was blank. If you turn it to AC it runs and if you turn the fan on it runs, it goes blank when you turn it to the heat setting. I removed the thermostat plate to inspect the wires and the thermostat itself, everything was tight, clean and together. I then followed the wires to the unit to assure everything was still connected and not damaged, fine again. Then took the cover off the furnace, to inspect the connections and motor, again everything is clean, connected and seems as it should be. I also check the breaker just-incase and of course it was fine. I don't know what else to check and I have been searching online for a couple hours getting no where fast. Also, for the responders who don't know the model, there is no battery to check in the thermostat and it is not programable. Just a simple digital temp display and an up and down buttons with two switches "system cool-off-heat and fan auto-on.

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  • Cheryl Handy
    Cheryl Handy Jul 20, 2014

    Amanda - I have the same model Honeywell. And the same thing happened. What did you finally learn?


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This model of Honeywell thermostat uses the 24v from the unit to power it. One of three things must have happened.

1. You have lost power to the furnace, therefore no power to thermostat.
2. You only have 4 wires hooked up on the thermostat and not 5. A lot of older homes had just 4 wire running to the thermostat, which will not work for this thermostat. You should have a wire connected to the (C) or (B) terminal on the thermostat, which ever one they have, I can't remember off hand. These are the Common terminals. They have to have the common from the furnace 24v power to operate.
3. Faulty thermostat, which will obviously have to be replaced.

Hope this helps and gets you to cooler days!

Jun 28, 2012 | Honeywell Heating & Cooling

1 Answer

My honeywell t8400c also went blank. I can not find a battery anywhere and if plugs into the wall to get electricity. Everything works but the digital read out. Any suggestions

it should take a battery and it has wires coming from the unit for power as well the stat splits open from the wall leaving some of it mounted on the wall the batteries are in the part that pulls off the wall and the wires are on the part that stays on the wall the power wire is red on r terminal

Sep 23, 2010 | Honeywell Electronic Programmable...

1 Answer

The thermostat worked correctly for months then

Go to
Under red welcome sign
Bottom red highlighted box literature/image search
Type in thermostat model number
Click on arrow to right
Then click on the PDF files owners manual / installation instructions
If you can’t find what you want, the bottom left and bottom of web
Page, they have homeowners assistance and Home products to get the help you need

May 10, 2010 | Honeywell Programmable Thermostat Heater

1 Answer

Honeywell T8400c thermostat went blank. central heating is not working although warm water is alright. any idea?

I would check the batteries in the thermostat. If the batteries are okay then you might have a blown fuse in the furnace or the furnace lost power check the breaker.

Jan 21, 2010 | Heating & Cooling

1 Answer

Honeywell thermostat t8400c intermittent blank screen

It actually sounds like the batteries are going dead if the 24v from the furnace are not providing the power to the thermostat.

Sep 28, 2009 | Honeywell Electronic Programmable...

1 Answer

New batteries ...display not working

remove the batteries and pull the metal tabs at each end stretching out a little the reinstall the batteries

Nov 26, 2008 | Honeywell Programmable Thermostat Heater

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