I am currently working on a combination AC/Furnace system Duo-Therm in a 8 yr old RV. The furnace fails to come on using the thermostat(but will if you jumper the wires before the ac board) The AC unit will only run if you put the fan on ON. I replaced the thermostat and AC board( its a kit you buy from Dometic). Nothing changed after installing the kit and I did check wiring and there is nothing I could find between the Thermostat and AC.