Its a Carrier split unit. Had the heater on yesterday and at some point when I wasnt around it turned off on its on. Its completely dead at the thermostat, and on the circuit board it doesnt get 24 volts either. So I tested voltage going into the transformer and it has 120 volts on all 3 wires going in but it only has .1 volts on the 24v side. Well thats not right. We have a 2 story house and both units have the exact same transformer so I measured the transformer on the working unit to be sure Im dong it right and it measures 120v on all 3 coming in, and about 27 volts going out. So I go and buy a new transformer, wire it up, and still no 24 volts! Did a resistance check, all wires have some resistance. So now Im stumped. If it has voltage coming in, how is it not coming out!? What should I check next?