Tempstar
Loss of 24V Control Power to Thermostat
Thermostat display is blank and furnace does not run.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Blown low-voltage fuse on Tempstar control board, Broken R or C wire to thermostat, Shorted thermostat or accessory, Failed 24V transformer
How to Fix / Troubleshooting
Safety: Turn off power before replacing fuses or working on wiring.
- Step 1 – Check furnace LED: If the control board LED is off, test the transformer as described earlier and replace if needed.
- Step 2 – Inspect board fuse: Replace a blown low-voltage fuse with the same rating. If it blows again, look for shorts in thermostat or accessory wiring.
- Step 3 – Test thermostat cable: Temporarily connect a short piece of new cable between furnace and thermostat. If the thermostat powers up, the in-wall cable is faulty.
- Step 4 – Check thermostat: Try a known-good thermostat to rule out internal shorts.
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Repair Difficulty
Medium
3/5
Required Part
Tempstar Furnace Transformer
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