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Last winter I had to have the mother board replace and the blower fan on my Carrier furnace and my a/c unit is a Fedders. The heat worked fine and still does. When I tried to turn on the a/c it wouldn't turn on. I tried to turn on the fan and it wouldn't turn on. Put it back on heat and it worked. Thought maybe the thermostated wasnt' working so I replace it and still have the same problem. There are 4 wires at the thermostat(red, white, yellow and green) and five wires at the furnace(two reds, a white, yellow and green). Only one red is hooked up at the furnace, the other is hanging in air. If I hook up the other red then the fan to the a/c unit turns on, but I have no cool air. Not sure where to go from here, any advice will help. Thanks,
 

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Typically the thermostat wiring goes as follows:
Red wire to R terminal
White wire to W
Yellow wire to Y
Green wire to G
If the thermostat has RH and RC terminals you need to jump them together.

The furnace is wired the same as above except the RH and RC don't apply.

Also at the furnace you should have a set of wires coming from the condensing unit. These wires are usually Red and White, but not always.
One of these two wires should be connected to the Y terminal along with the yellow wire from the thermostat. The other wire is connected to the C or Common terminal. Hope this helps.
 
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