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HIGH URGENCY

Carrier Code 24 — Secondary Voltage Fuse Open

Code 24 (2 short + 4 long flashes)

Secondary voltage fuse blown — short in 24V control wiring.

Brand: Carrier (Also covers Bryant, Payne)  |  Equipment: Gas Furnace

SAFETY: Don't keep replacing fuses without finding the short. Higher-amp fuses bypass safety and can cause fires.

What This Code Means

Code 24 means the 3A or 5A fuse protecting the 24V secondary side of the transformer has blown. Almost always caused by a short circuit somewhere in the 24V wiring (thermostat, valves, switches).

Likely Causes (Ranked)

Most common

Short in thermostat wiring

What: Damaged or pinched thermostat wire creating short to chassis.

How to check: Disconnect thermostat wires at furnace; replace fuse; if it doesn't blow, the short is in the thermostat circuit.

Fix: Locate and repair short, replace fuse.

Shop parts
Common

Short at gas valve or other 24V device

What: Failed coil in gas valve, contactor, or other 24V device causing a dead short.

How to check: Disconnect each 24V device one at a time; replace fuse and test.

Fix: Replace the failed device.

Shop parts
Less common

Damaged control board

What: Board internal short blowing the fuse.

How to check: Tech diagnostic.

Fix: Replace control board.

Shop parts

Diagnostic Steps

  1. Locate the secondary fuse on the control board
  2. Replace with same amperage (usually 3A)
  3. If it blows again immediately, you have a short
  4. Disconnect 24V devices one at a time to isolate
  5. Call HVAC tech if you can't find the short

Frequently Asked Questions

What fuse does my Carrier furnace use?
Most use a 3A automotive-style blade fuse on the control board. Some use 5A. Match the existing rating exactly.
Why does my fuse keep blowing?
A continuing short circuit. Don't keep replacing without finding the cause.
Can I fix this myself?
Yes for simple thermostat wire shorts. Call a tech for anything beyond that.

Related Carrier Codes

Carrier Error Code 21

Related Symptoms

Furnace Not Igniting

Need More Help?

Try our interactive symptom diagnostic tool or browse Carrier parts.

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