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Digital thermostat not working

We have a Honeywell digital thermostat (# on tag on back of the unit are...
T8400C 1024). I noticed today that our fan wasn't running for awhile when I went to check the thermo. to see if it was on auto or on, the screen was blank. I tried to turn on the furnace and ac but nothing is working.

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  • boberding Sep 15, 2008

    I just figured that was obvious... the unit doesn't have batteries. As it turns out it was a problem with the circuit board but thanks for the insightful comment.



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Can you saY, Batteries!

Posted on Sep 15, 2008

  • Anonymous Apr 26, 2016

    No, but I can say you're a dumbazz because this doesn't use batteries!



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1 Answer

The display on my thermostat T8400C 1024 isn't working. Everything else works but the heat and the flame is not igniting either. Please help.

Sounds like a manual limit needs to be reset (thermal disc). Generally the manual limits are around the exhaust - backdraft, or burner assembly - flame rollout.

Feb 15, 2014 | Honeywell Heating & Cooling

1 Answer

Honeywell thermostat t8400c

Probably no power (24 volt) to the unit. Check 3amp fuse in the air handler. Also check that it's wired properly.

Sep 19, 2013 | Heating & Cooling

1 Answer


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

Changed honeywell T8400C 1024 to Hunter 44760, I have five wires white, red, yelliow, green, blue. Hunter Thermostat reads "NO A/C Power.

On the t-stat and the furnace make sure the wires are hooked like this...

Red to R
White to W
Green to G
Yellow to Y
Blue to C

then it should work..

Thermostat Wiring and Terminal Designations


Jun 12, 2011 | 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

I have a Honeywell T8400C 1024 thermostat. the last two nights, our heat continues to run without shutting off at the pre-set temperature. We have woken up to 81 degrees in our house. What is the...

Try taking the batteries out for 5 min,or disconnect power for 5min.
To clear it out.
Then re input all your times days and temps,

Is t stat in a cold location near a door or window?this could also override your settings,most progrmamble stats will go into override at 58.f im guessing this is not your problem.

If it continues after you reset,i would consider a new thermostat.

Hope this helps.

Jan 28, 2010 | Heating & Cooling

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...

2 Answers

My thermostat quit working

Does it have batteries that need replacig. I think so. good luck.

May 31, 2009 | Honeywell Programmable Thermostat Heater

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