This time of year, I get a good number of questions about furnace repair, a common winter task for anyone who performs RV repair. The first thing do-it-yourselfers want to do is replace the thermostat. It is, after all, right there handy and not too expensive. The only problem is, it is almost never the thermostat and it is very easy to know if it is.
First, if the furnace blower runs, leave the thermostat alone. An active blower means the thermostat is powering the blower relay, end of story.
Second, suppose you turn up the thermostat and nothing happens, what then? Naturally the initial step is to check the fuse for the furnace. You will find it in the breaker box with the other fuses.
If the fuse is good, take off the panel where the furnace is located so you can see the wires to the furnace. You will find a red lead, a yellow lead, and two blue leads. Using a multimeter, check for power between the red 12 VDC+ and the yellow 12 VDC-. You will need more than 10 VDC there or the furnace will not operate. If you have a good fuse and no power, you have a break in the wires somewhere. Usually that means running new wires as getting to the old ones is seldom possible. Running wires in cabinets is a common solution.
If you have power between the red and yellow wires, disconnect the two blue leads from the red and white thermostat wires at the furnace. Be sure to mark them so you know what goes where when you put it all back together. Now tie the two blue leads from the furnace together. Doing this bypasses the thermostat. If the furnace now fires and runs, replace the thermostat. If nothing happens the problem is at the furnace.
Remember, every time you troubleshoot you are simply answering a series of questions and one set of questions always leads to the next. Break diagnosis down into steps and repair is much simpler.
Steve (Mobility RV Service 423.341.8792)