How to Troubleshoot a Miller Thermostat

Turn off the power to the thermostat and remove the screws on the sides and corners of the thermostat and remove the thermostat faceplate.

Check the wall opening for any hot or cool air that may be blowing through. If there is, it can effect the thermometer of the thermostat and may be the reason the thermostat is turning on and off at odd times.

Caulk the frame of the wall opening to ensure that no excess heat or cold gets to the thermostat.

Use a small brush to dust all around the thermostat. An accumulation of dust could be the reason your thermostat isn't functioning properly.

Replace the thermostat and turn the power on. Place a thermometer next to the thermostat and check the temp of the thermostat with the temp on the thermometer. If they don't match up the thermostat thermometer may be faulty and you may need to replace the thermostat. If this isn't the issue, proceed to the next step.

Turn off the power to the thermostat once again and remove the faceplate.

Check the wiring for corrosion. If the wires are corroded, unplug them from their terminals at the rear of the thermostat clean them with a cloth and cut about 1/2 inch of sheath from the ends with your wire cutters. Then re-connect the wires to the terminals.

Turn on the power to the thermostat as well as to your voltmeter. Set the voltmeter to "V" and locate the transformer that the thermostat wires connect to.

Touch the red lead on the voltmeter to the terminal on the transformer and wait for the voltmeter to produce a reading. If it doesn't the transformer is likely faulty and will need to be repaired or replaced.

Replace the faceplate of the thermostat.