Furnace LED shows code 24; blower may run but no heat, or furnace may be completely inoperative.

Safety first: Turn off furnace power before accessing the control board. Do not bypass or oversize the fuse.

• Locate and inspect fuse: Remove the blower door and find the control board. Identify the low-voltage blade fuse (typically 3A or 5A). If the metal link is melted or glass is dark, the fuse is blown.
• Check thermostat wiring: Inspect the thermostat cable where it passes through the furnace cabinet and along framing. Look for crushed, pinched, or bare wires touching metal.
• Disconnect external loads: Temporarily disconnect wires going to outdoor A/C (Y), humidifier, or other accessories at the furnace control. Replace the fuse with the exact same rating, restore power, and call for heat.
• Isolate the short: If the fuse holds with accessories disconnected, reconnect them one at a time until the fuse blows again. The last reconnected circuit is likely shorted.

If the fuse blows immediately even with only R and W connected, the control board or internal wiring may be shorted and require professional service.