24
Comfortmaker Secondary (24V) Fuse Open / Low-Voltage Short

LED flashes code 24; furnace may not start or may show low-voltage issues.

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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 3–5A automotive-style fuse on Comfortmaker control board, Shorted thermostat wire to metal, Shorted humidifier or A/C contactor wiring, Miswired accessory to R and C

How to Fix / Troubleshooting

Safety: Turn off power before replacing any fuses. Never bypass or upsize the fuse; this protects the control board.

Steps:

  • Locate the fuse: On most Comfortmaker boards (HK42FZ, CEPL130438), a small automotive-style blade fuse (3–5A) protects the 24V circuit. Inspect it visually for a broken element.
  • Replace the fuse: With power off, replace with the exact same amperage rating.
  • Inspect thermostat and accessory wiring: Look for bare wires touching the furnace cabinet, especially near the blower compartment, at the thermostat, and at any connected humidifier or A/C contactor.
  • Test operation: Restore power. If the new fuse blows immediately, disconnect thermostat wires at the furnace and try again. If it holds with wires removed, the short is in the field wiring or thermostat.

When to call a technician: If the fuse continues to blow even with all external wires removed, the control board or internal transformer may be shorted and should be professionally replaced.

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Repair Difficulty

Medium 3/5

Required Part

24V Control Board Fuse
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