Carrier 59SC2B Error Code 24: Secondary Voltage Fuse Is Open
What Does Code 24 Mean?
Status code 24 on the Carrier 59SC2B indicates the secondary voltage fuse is open. This small fuse on the control board protects the 24-volt circuit — the low-voltage side that powers the thermostat connection, the transformer secondary winding, and any connected accessories such as humidifiers or zone damper panels.
The fuse blows when it detects excessive current draw, which almost always results from a short circuit in the thermostat wiring. Common causes include bare thermostat wires making contact inside the wall, a pinched wire behind the thermostat mounting plate, rodent damage to wire insulation, or incorrect wiring during a thermostat replacement. Damaged or pinched wires can also cause this, especially in older installations where wire insulation has become brittle.
Simply replacing the fuse will not solve the problem — the short circuit must be located and repaired first. A technician will systematically disconnect thermostat wires at the furnace and test each conductor for shorts before replacing the fuse.
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 — locating short circuits in thermostat wiring requires systematic testing with professional equipment
✓ Verified against manufacturer service manual — March 2026