Question about Honeywell Programmable Thermostat Heater

3 Answers

Honeywell ST6400C with no display

Hi, I just returned from holiday to find no display on my Honeywell ST6400C. When i went on holiday I turned the heating off but have done this before. Is this an issue with a fuse? The gas is working along with all the electrics in the house. I followed the instructions and pressed the reset button but nothing worked. Thanks

Posted by on

  • gmelville Nov 17, 2008

    Thanks for your help

  • Anonymous Jan 03, 2009

    yes we had the same problem over a holiday. The elctricity supply to the programmer was switched off. Bad idea! The solution proposed seems not appropriate. The st6400 is the programmer rather than the thermostat. The usual solution is to press the RESET button. But if this doesnt work how about replacing with the newer st9400? Circa £50-£65.. Same backplate and wiring.. (how about leaving connected to mains over holiday..?)


3 Answers

  • Level 1:

    An expert who has achieved level 1.


    An expert whose answer got voted for 2 times.

  • Contributor
  • 1 Answer

I pressed the reset button in error and the display just would not return o no heating or hot water. Nothing tripped on the board and all wires and the unit itself showed power coming through.I got it working again by sheer determination.

What I did
I took the unit from the base plate and opened the front panel to see circuit board. It has 4 clips and it all goes back together easily.

What I did.Time taken 15 minutes.

The tiny rubber reset button was collapsed, perhaps maintaining the 'off' situation.

I moved a tiny wire from touching the side of the battery.Perhaps not important but it didnt look right.

When i remounted the unit it lit up beautifully.

So the solution lies in 1 or all of these 3.
If it happens again I will try

1. simply remove the unit from its base plate and remount to see if that is all it takes.

2. Open the front and check the reset button isnt collapsed and stuck.

3. Just move that little wire from leaning against the battery -(least sure of why this might help)

hope this might help someone else

Catherine - a determined blonde lady

Posted on Mar 04, 2011

  • Level 1:

    An expert who has achieved level 1.


    An expert that has over 10 points.


    An expert whose answer got voted for 2 times.

    Problem Solver:

    An expert who has answered 5 questions.

  • Contributor
  • 17 Answers

I am not familiar with this particular model but in general all digital thermostats are either powered by batteries or by the 24 volt transformer from the furnace. Usually the standard heat/cool type that you may have is powered by batteries and will show a low battery display for a few days before they go out entirely. You may have missed that warning because of your being away.
There will either be a small door on the front of the thermostat or you will need to pull the thermostat from the subbase and the batteries will be inside. Usually 2 AA size....replace with the good kind.

Good luck

Posted on Nov 14, 2008

  • Level 1:

    An expert who has achieved level 1.

  • Contributor
  • 1 Answer

Not sure that the advice above is appropriate. TheST6400 is a programmer rather than a thermostat.. we had exactly the same problem after a holiday exceeding the specified four day back up so the usual solution of pressing the RESET button didnt work. We are replacing ours with ST9400 honeywell computer that has the same backing plate and wiring. The answer seems to be, once installed do not switch off at the mains.

Posted on Jan 03, 2009

Add Your Answer

Uploading: 0%


Complete. Click "Add" to insert your video. Add



Related Questions:

1 Answer

Can I replace the battery in my Honeywell St6400c central heating controller so it is ok in power cuts? If so how?

Unit manual will tell you how to open - remove old battery and replace with new one.

Sep 27, 2016 | Thermostats

1 Answer

My st6400c Honeywell programmer blank. Mains turned of whole day. Now back on, but st6400c blank.

Have you tried pressing the 'Reset' button (with the tip of a pencil)? After doing this, you will have to re-enter the various times. It could be that your rechargeable battery is nearing the end of its useful life, by the way.

Feb 23, 2011 | Honeywell Programmable Thermostat Heater

2 Answers

The battery in our Honeywell ST6400C programmer needs replacing as it no longer gives us any back-up; what is the battery, please? Mark Houston

Hi shastalaker

Thanks for the info but how to get the front cover off. Two screws underneath allow the whole unit to be removed but then - nothing. There are two plastic tabs but they do not release anything as they are at one end. The other end remains firmly fixed.

Any thoughts


Jan 18, 2010 | Honeywell Programmable Thermostat Heater

1 Answer

St6400c thermostat wont run the central heating independantly

This does not sound like a thermostat problem. I would be looking for a zone valve or circulator pump problem.

Nov 05, 2009 | Honeywell Programmable Thermostat Heater

1 Answer

Honeywell ST6400C - hot water not working

some digital thermostats do no not work with zone valves is this a new thermostat that you had installed

May 08, 2009 | Honeywell Programmable Thermostat Heater

2 Answers

Replacing Honeywell heating progrmmer

yep the wires will hook up to the same designations, just write down what color was connected where. You will need to set the thermostat up for your particular situation...gas, electric, heat pump, ect. The installation maunel is very clear on this aspect so you should not have a problem.

Oct 30, 2008 | Honeywell Programmable Thermostat Heater

1 Answer

No hot water honeywell st6400c

HW valve was stuck in the open posiion and the micro-switch was not working so thats why boiler and pump were not starting. Bled system to below the level of the valve to be changed, and replaced valve with a drayton ( easy quick change of valve head if needed no screws )

Sep 07, 2008 | Honeywell Programmable Thermostat Heater

1 Answer

Honeywell ST6400C programmer

Hi yes sound like it not working on hot water only it would check ou the wiring on this side

Jul 25, 2008 | Honeywell Programmable Thermostat Heater

Not finding what you are looking for?
Thermostats Logo

2,090 people viewed this question

Ask a Question

Usually answered in minutes!

Top Honeywell Thermostats Experts

Bill Long

Level 3 Expert

577 Answers


Level 3 Expert

1875 Answers

Brandon Berry
Brandon Berry

Level 2 Expert

407 Answers

Are you a Honeywell Thermostat Expert? Answer questions, earn points and help others

Answer questions

Manuals & User Guides