How do I test to see if a thermostat is operating? The furnace will not shut off when you lower the thermostat. When you trip the reset, the flame will shut off, but the fan continues to run to cool the heat chamber. I suspect the thermostat. This is an old oil furnace that I use infrequently in the garage. It has operated fine until this

Disconnect the red wire on the thermostat and that should shut down the furnace if the thermostat is for some reason stuck on.