Carrier 59SC5A Error Code 24: Secondary Voltage Fuse Is Open
What Does Code 24 Mean?
Status code 24 on the Carrier 59SC5A indicates that the secondary voltage fuse on the control board is open. This fuse protects the 24-volt low-voltage circuit that connects the control board to the thermostat, transformer secondary, and any accessories like humidifiers or zone control panels.
A blown 24V fuse is caused by a short circuit somewhere in the low-voltage wiring. The short can occur at the thermostat (bare wires touching behind the wall plate), in the wire run between the thermostat and furnace (pinched or rodent-damaged insulation), or at an accessory connection. Damaged or pinched thermostat wires are a particularly common cause in installations where wires pass through tight spaces.
Replacing the fuse without finding the short will cause it to blow again immediately. A technician must disconnect all low-voltage wires at the furnace and test each one individually for shorts to ground and to other conductors. Once the short is located and repaired, the fuse can be replaced and the system verified.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Short circuit in thermostat wiring | Most common | ✗ Call a pro → |
| Damaged or pinched thermostat wire | Common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- When this code appears — systematic short-circuit tracing in thermostat wiring requires professional diagnostic tools
✓ Verified against manufacturer service manual — March 2026