I have an old Honeywell CT3400A1007 thermostat. Every winter when I first go to turn it on, the heat won�t work. We have found that sometimes opening the furnace door and then replacing it and waiting a while works, along with various other inconsistently successful tricks. Once it gets going it is usually fine except for once or twice all winter. A few months ago our tech finally decided it was one of the channels, I believe the W, in the thermostat. He said it just needs to be replaced. For a variety of reasons, I prefer not to.

I did some research and here is what I find. There are 4 wires, white, green, yellow and red, connected to W, G, R and Y, of course. I used my digital multimeter to check voltage. If I touch the red probe to R and then the black to the other terminals, I get readings of 27.5, except for the W which reads 12.5 or so. So I assume this confirms a wiring problem.

Just to be clear, when putting on the heat, neither the fan nor the furnace comes on. The AC and fan work fine otherwise. The furnace is not the issue, and we even had a new mother board installed recently just in case. The W terminal reads the same no matter if the thermostat is set to Off or to Heat. And for right now, it is not working. I can touch any of the white wire that is exposed, not just the screw, and still get 12.5 V.

Do I just take the thermostat off the wall and maybe clean the wiring and replace it? Could the problem be in the thermostat itself? If so, can it be fixed? I assume the latter is the case. I don�t want to start pulling it off the wall or apart until I have some idea what I am looking for in there. If I remember correctly, I think the tech was able to bypass the W terminal to get it on, but I am uncertain of this or the details. Again, I am not trying to save money. I just don�t want a new thermostat.

Thanks for any suggestions.