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. enter image description here New Thermostat wiring. enter image description here Furnace Control wiring. enter image description here

  • The wiring in the furnace... Was that how you found it, or did you change it? – Tester101 Jun 12 '15 at 2:56
  • There should be a cable running from the furnace to the outdoor unit... Where is that connected? What is the make and model of the furnace? Is there a part number printed on the furnace control board? – Tester101 Jun 12 '15 at 2:59
  • Never change the wiring of the furnace. Couldn't figure out the cable to the outdoor unit you mentioned. It is a Carrier 58STA090---10114. – John Jun 12 '15 at 20:04
  • The yellow wire on the thermostat reads about 18V, but the Y terminal on the furnace reads about 28V. So I am thinking there might be something wrong the cable. – John Jun 12 '15 at 20:06
  • 18 volts between Y and what (where is your other probe)? Is the outdoor unit controlled through the furnace (e.g. does the furnace blower run when the A/C is on)? Do you have a multizone system, and if so, do you have a zone controller? – Tester101 Jun 12 '15 at 23:43

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