Question about Honeywell Heating & Cooling
Fault - idle
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Posted on Jan 02, 2017
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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Page 12 of the user manual explains this:
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