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Hi, make sure the thermostat is switched to “heat” and turn up the temperature so there is a call for heat. The 12VDC that powers (or switches through the thermostat) typically comes through the furnace unit. If you have a thermostat with its own batteries (does not use the coach 12vdc for power) make sure the batteries are good. If you have a multimeter use the voltmeter part and see if there is 12vdc on the thermostat R or RH labelled terminal. You will probably need to carefully open the face portion of the thermostat to get at the labelled wiring terminals. If there is no 12vdc on the thermostat R or RH terminal you will need to open up the outside furnace unit door (described by previous poster) and determine where the 12vdc has failed. You should have a manual for your furnace that has a schematic.....if not search for it on line. Normally when the thermostat calls for heat the control board in the furnace will start the furnace fan motor and when it starts blowing air the sail switch operates (it's located inside the fan area in the furnace) which tells the control board that the furnace fan is working properly so the remainder of the furnace ignition sequence can continue. If the furnace fan does not run as you say, you need to look for where the 12vdc feed has failed/opened. (Fuse, breaker marked furnace, bad wire connection etc. The multimeter is an essential and inexpensive tool that every RVer should own and learn to use).
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