This winter the furnace control board in my Lennox Gas Furnace / AC unit failed and I had it replaced by a contractor. The contractor mis-wired the thermostat inputs to the board. He wired the Green (fan) wire to the C (compressor connection) board input and the C wire to the Green (fan) board input. He properly wired the other end of the C (compressor connection wire to the yellow (A/C) input.

In this configuration the gas heat worked properly all winter (I was then unaware of the mis-wiring). But once I switch the thermostat to A/C, my problems began. At this point the 3 amp break on the board opened and everything went dead (as it should). So I found the mis-wiring issue and re-wired the inputs correctly, then I reset the 3 amp breaker on the board.

Now... the only thing that works is the fan on "on" mode. When I put the fan in auto and try to run the Heat or the A/C... nothing happens. The board LEDs show the flashes for "normal operation" but I can't get either the A/C or the Heat to start up now...

Are there any other fuses or breakers on a furnace/AC unit like this that I should be checking? Is the Board failing even though the LED show everything should be running OK? Did running 24V down both sides of the compressor do something that would prevent both the furnace and A/C from now working? Thanks in advance for any help you can provide.

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Originally Posted by SteveGe

This winter the furnace control board in my Lennox Gas Furnace / AC unit failed and I had it replaced by a contractor. The contractor mis-wired the thermostat inputs to the board. He wired the Green (fan) wire to the C (compressor connection) board input and the C wire to the Green (fan) board input. He properly wired the other end of the C (compressor connection wire to the yellow (A/C) input.

In this configuration the gas heat worked properly all winter (I was then unaware of the mis-wiring). But once I switch the thermostat to A/C, my problems began. At this point the 3 amp break on the board opened and everything went dead (as it should). So I found the mis-wiring issue and re-wired the inputs correctly, then I reset the 3 amp breaker on the board.

Now....the only thing that works is the fan on "on" mode. When I put the fan in auto and try to run the Heat or the A/C... nothing happens. The board LEDs show the flashes for "normal operation" but I can't get either the A/C or the Heat to start up now....

Are there any other fuses or breakers on a furnace/AC unit like this that I should be checking? Is the Board failing even though the LED show everything should be running ok? Did running 24V down both sides of the compressor do something that would prevent both the furnace and A/C from now working? Thanks in advance for any help you can provide.

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I do not work on many Lennox units but I have a feeling the C that you mentioned means common, from the transformer.

If you just have an AC and not a heat pump you should be using two small wires coming from the condensing unit, the color does not matter, one should be on the yellow terminal and the other should bo on the common terminal, then if the colors are right coming from the thermostat you should have a red wire ont the R terminal, A white wire on the W terminal, The green on the G terminal ( which sounds like that is the way they are), that leaves the yellow wire that should go on the Y terminal with one of the wires that came from the condensing unit.

If this answer helps please comment rate it below.

You would do well to go to the section of the sticky at the top of the H&AC forum on thermostat wiring, and also do I have a heat pump. I don't see how connecting both condenser coil terminals to the same side of the transformer should have hurt. There is also a section on checking if the problem is the thermostat or furnace.