24v I believe. When detached at the thermostat the dial barely moves but it does a little.

You need to test it but sounds like a short in T-stat wire. Pull new wire once you confirm.

Then you may want to set up some mice traps. The plastic coating on those wires, mice love that stuff. You'd think they would learn after getting shocked, but no they will come right back and chew even more.

Even worse if it turns out to be squirrels.

I service the Katy, Texas area.

When you say "furnace kicked on" do you mean the gas valve ignited and heat was coming out of the furnace?

If you detach all the wires, you can measure the voltage between on each thermostat terminal and the C terminal to see if the terminal is being energized (24 V) correctly when turning on the fan to auto and on, and calling for heating and cooling. If it is not then it is a thermostat problem. If thing are correct then connect the wires and repeat the measurements. If you measure changes then it is likely a wiring problem.

Thanks!

Yes, the gas valve ignites and heat comes out of the furnace.

When you say detach and test each wire, I assume you mean at the furnace end? Also, does it matter than my thermostat wire does not have a C wire? There is a wire coming off of the c terminal but not from the thermostat.

When I test between the wire from the thermostat and the C terminal, should I only see 24V on the heat wire when the thermostat is set to heat and when the temperature is below the set temperature?

I have an update. I am thinking it is a Thermostat wire problem even though I have not totally verified it. I ran a test by attaching the old thermostat directly, using a fresh thermostat wire and everything worked correctly. So I figured I had verified that it is the wire. I followed the wire and pulled and tugged and jiggled it as it winds all over the place. The old thermostat wire goes into the furnace and is spliced to another thermostat wire that is only a couple feet long, which is attached to the circuit board. I next tried attaching the old wire coming from the new thermostat directly to the circuit board and this worked, so I thought I had isolated it to the shorter wire inside the furnace. But when I tried hooking everything back up the way it was, it all works correctly now. So I think by moving and pulling and reattaching all the wires, that somehow undid the short at least for now. I do want to run a new 5 wire from the thermostat at some point but for now things are working. :-)

You should only read approximately 24V AC between the W and C when the thermostat is calling for heat. The same is true for Y and C when calling for cooling. Setting the fan to the on position will cause the G terminal to be at 24 V.

Intermittent wiring shorts can be hard to find. I am glad you were able to resolve it.