Dur-therm furnace heat controll with Dometic remote

sounds like thermostat issue not communicating with relay modeule

sounds like communication cable from t-stat to board bad or indoor temperture sensor failed.

I assume it's remote control. (comfort control) Have you tried following procedure for selecting "Mode"?

I think that I would change out the thermostat only due to the symptoms you are describing. Think the control board may be fine however, the thermostat sounds as if the internal componants are going out. Good luck on your repairs

Dave

Make sure fan relay is wired correctly (G) wire. Ensure the fan on therm is not on, it should be on Auto or off.

If your furnace is short cycling because it has to keep heating your home, the delay in the control board may be staying on to compensate for the exchangers getting hot. If it heats normally, then put your attention to your thermostat.

Set the heating system on aux heat only. Fan should not be on, if it does stay on after a heating cycle, see if the furnace is heating normally, and reaching temp as desired before the heat comes off. Then check the therm wiring, pull the face of the therm, disconnect green wire (fan) and try the heating again.

Checked all breakers & fuses in panel ?

Sounds like a stuck relay on the unit itself. Maybe there is a short in the control wire that comes from the thermostat that activates the fan.

The thermostat probably has one hot wire at all times. The thermostat then chooses which wire to send the voltage down based on your selection. One of those wires is probably for the fan only. Make sure that it is in good condition (no bare, pinched or wires)