my old thermostat stopped working, so I purchased a new one to replace that. The old one was powered by two AA batteries. A green wire connected to G, a white wire connected to W, a blue wire connected to Y, a red wire connected to RC, and a red wire connected from RC to RH, a yellow wire was not connected to anything. Everything was working fine (heat, fan and AC).
I also openned the furnace and looked at the control board, and all wires are connected to terminals according to color codes, that is, green to G, white to W, yellow to Y, red to R and blue to Com.
The new Honeywell thermostat I bought is not powered by batteries, so it requires power from the furnace. So I hooked up the wires to the new thermostat according to the color codes (matching what is on the furnace board, also as instructed by the thermostat mannual). This is different from the old one, this time, yellow to Y and blue to C. The thermostat is on, the heat works, but AC is not. The compressor outside is not running at all. So I switched the yellow and blue wires on the thermostat, the thermostat is not turning on. I used a multimeter to test all wires, all wires have about 28V, except the yellow wire has only about 18V.
What could be the problem? Could the yellow wire be broken? Can I connect both yellow and blue to Com terminal in the furnace control board? Thanks in advance.
Old Thermostat wiring. New Thermostat wiring. Furnace Control wiring.