Furnace LED shows code 24 (secondary voltage fuse is open), no operation, thermostat appears dead.

Safety first: Turn off power to the furnace before replacing fuses or handling wiring.

• Locate and inspect fuse: On most Carrier control boards, find the 3–5A automotive-style blade fuse. Remove and inspect; if the metal link is broken or blackened, it is blown.
• Check for obvious shorts: Inspect thermostat cable where it enters the furnace cabinet and along any sharp metal edges. Look for bare copper touching the cabinet or other wires. Repair damaged sections with wire nuts or replace the cable.
• Disconnect accessories: Temporarily disconnect wires going to external devices (humidifier, air conditioner contactor, zone controls) from the board's HUM, Y, or other terminals. A short in these circuits can blow the fuse.
• Replace fuse: Install a new fuse of the exact same amperage rating. Do not bypass the fuse or use a larger size.
• Restore power and test: Turn power back on. If the new fuse blows immediately, there is still a short that needs professional diagnosis.