Symptoms
- Thermostat is completely blank
- House power and breakers are fine elsewhere
- Problem started suddenly, sometimes after a thermostat wiring change
Likely causes
- CommonShort circuit from a wire touching the wrong terminalA stray wire strand or miswiring is the most common cause of a blown fuse.
- PossibleOld or failing thermostat causing a shortAn internal fault in the thermostat can draw excess current.
- PossibleFaulty low-voltage transformerA failing transformer can blow fuses repeatedly even after replacement.
- PossibleShort elsewhere in the thermostat wire runDamage to the wire inside a wall or attic can cause a short far from the thermostat.
What to do
- Turn off power to the furnace or air handler at the breaker before doing anything else.
- Open the furnace or air handler access panel and locate the small glass tube fuse near the control board.
- Note the fuse's amperage rating, usually printed on the board or the fuse itself.
- Replace the fuse with an identical replacement rated for the same amperage.
- Before restoring power, check the thermostat wiring for any bare wire strands touching each other or the wrong terminal.
- Restore power; if the new fuse blows again immediately, stop and call a professional, since there's likely an active short.
Tools and parts
- Replacement fuse (same amperage)
- Screwdriver
- Flashlight
Furnace control board fuseSearch: “3 amp automotive glass fuse thermostat”
When to call a pro
Call an HVAC technician immediately if a new fuse blows again right away, since there's an active short that needs to be traced.
Safety
Use caution
Always cut power at the breaker before opening the furnace panel or handling fuses.