Hi,

I'm trying to replace an old mercury thermostat with a programmable digital one. It is for a cooling only system. It has three wires, black, blue and white. I have connected them the way they were connected before, White to RC, Black to Y, and Blue to G. The system turns on, but it won't follow the program, i.e. it doesn't turn off when the desired temperature is reached. I'm not sure if the problem is the wiring or the thermostat. Any help would be appreciated.

Thanks.

Probably programming or setup. Try jumping Rc to Rh.