A little backstory should help... my parents built the house in 1996 with no central air. They added central air in 1999 and it was fine till three years ago. We had a programmable thermostat installed and once a year now for three years, we have had a service tech out to fix the thermostat because the AC would not turn on. It would be fixed and then the heat wouldn't come on in winter.
The wires are six wires:
I hooked yellow to the y terminal in the thermostat and nothing happened. I checked it at the furnace and there is 24 volts at the wire, but its not hooked up to anything. The orange is not hooked up either, I believe this is reserved for a heat pump.
In the thermostat, white, green and red are hooked up, but blue, yellow and orange were not.
There are four colors out of the furnace these are hooked to:
They are not matched color to color and there are jumpers between them.
I have attached pictures showing the wiring at the thermostat and I suspect that one of the other colors, probably red are supplying power to the AC, but don't want to risk shorting something.