24
Day & Night
24V Control Circuit Open/Short
Furnace LED flashes code 24; no response to thermostat, or intermittent operation. Often indicates open or shorted secondary (24V) circuit.
Advertisements
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.
Advertisements
Possible Causes
Blown low-voltage fuse on control board, shorted thermostat cable, miswired external safety device (e.g., float switch), shorted humidifier or AC contactor coil
How to Fix / Troubleshooting
Safety first: Turn off furnace power before inspecting wiring.
- Check control board fuse: Remove the blower door and locate the integrated control board. Inspect the 3A or 5A automotive-style blade fuse on the board. If blown, replace with the same rating only.
- Isolate thermostat wiring: With power OFF, disconnect all thermostat wires from the R, C, W, Y, G terminals. Restore power and jumper R to W at the board. If the furnace runs normally, the thermostat or its wiring is likely shorted.
- Inspect external devices: If you have a humidifier, float switch, or AC connected, inspect their low-voltage wiring for bare spots or pinches against metal.
- Replace damaged cable: If the thermostat cable sheath is cut or chewed, replace the cable with new 18/5 thermostat wire.
If the fuse continues to blow with all external wiring removed, the control board itself may be shorted and require replacement.
Advertisements
Repair Difficulty
Medium
3/5
Required Part
Low-Voltage Fuse on Control Board
Check Price on Amazon
We may earn a commission from links on this page.
Advertisements