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St9400c fault idle

Fault - idle

Posted by Anonymous on

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Posted on Jan 02, 2017

  • 1 Answer

SOURCE: internal fault

My installer hadn't the foggiest, lots of posts on the internet about this, and no solutions, so here's one I just got to work. Turn off mains to unit. (Note display still shows "INTERNAL FAULT" - this tells us message is maintained by the internal battery.) So, take ST9400C off the mounting plate, and using a small flat blade screwdriver ease out the four clips holding the plastic housing, top/bottom/sides. Now you can see the circuit board, in the middle of which is the lithium cell - ease the copper clip off to take this cell out. Display still says "INTERNAL FAULT" so this tells us a capacitor is maintaining the memory. Noting the large capacitor (blue in my unit) bottom right of the circuit board, short out the wires going to it (by putting your small flat blade screwdriver across both wires). Display no longer shows fault message. Put lithium cell back in, put housing back together, remount unit on backplate, turn on mains supply and unit starts as from new out of the box. Reprogram. Sorted. (I could perhaps have left the capacitor to discharge naturally once the cell was out, but life is too short and I wanted my heating to work.) Hope this helps in the worst UK winter for ages.

Posted on Jan 07, 2011

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My honeywell st9400c changing from auto to continuous


How old are the batteries? You may want to replace them to see if it helps. If it doesn't then get a new thermostat. Nothing inside the AC/Heater will cause that.

Mar 24, 2014 | Honeywell Heating & Cooling

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Replacing an ST699 with a ST9400C following repl wiring diagram, but programmer does not come on. Is the diagram incorrect? Wiring colours do not follow any of the plans. Syst has room stat, cyl stat,...


Disregard wire colors. Instead wire by terminal designation. In other words :
Wire on "R" Terminal on old thermostat to "R" Terminal on new thermostat no matter the color.

Jun 14, 2011 | Honeywell Heating & Cooling

1 Answer

Mine is a Honeywell st9400c also (same as "andyrobbens") and it also shows "INTERNAL" (not internal fault, just the word "INTERNAL") Switching power off is not resetting it,...


By the way the battery is a common CR2032 battery and can be swapped out in 2 minutes. Turn off the unit at the mains, 2 screws at the bottom, push in the plastic connectors and the battery can be slid out

Jan 15, 2011 | Honeywell Electronic Programmable...

1 Answer

Internal fault


My installer hadn't the foggiest, lots of posts on the internet about this, and no solutions, so here's one I just got to work. Turn off mains to unit. (Note display still shows "INTERNAL FAULT" - this tells us message is maintained by the internal battery.) So, take ST9400C off the mounting plate, and using a small flat blade screwdriver ease out the four clips holding the plastic housing, top/bottom/sides. Now you can see the circuit board, in the middle of which is the lithium cell - ease the copper clip off to take this cell out. Display still says "INTERNAL FAULT" so this tells us a capacitor is maintaining the memory. Noting the large capacitor (blue in my unit) bottom right of the circuit board, short out the wires going to it (by putting your small flat blade screwdriver across both wires). Display no longer shows fault message. Put lithium cell back in, put housing back together, remount unit on backplate, turn on mains supply and unit starts as from new out of the box. Reprogram. Sorted. (I could perhaps have left the capacitor to discharge naturally once the cell was out, but life is too short and I wanted my heating to work.) Hope this helps in the worst UK winter for ages.

Jan 28, 2010 | Honeywell Electronic Programmable...

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