Carrier 59SC2B Error Code Code 24: Secondary Voltage Fuse Is Open
What Does Code Code 24 Mean?
Code 24 on the Carrier 59SC2B indicates that the secondary voltage fuse is open — meaning the small 3-amp automotive-type fuse located on the furnace control board has blown. This fuse protects the 24VAC low-voltage circuit that powers the thermostat, control signals, and accessories like humidifiers or zone dampers.
The fuse blows when there is a short circuit somewhere in the 24VAC wiring. Common culprits include damaged thermostat wires (often pinched during construction or chewed by rodents), a shorted thermostat, or a faulty low-voltage accessory. The manual recommends disconnecting the thermostat leads from the furnace to help isolate whether the short is in the thermostat wiring or within the furnace itself.
The 3-amp fuse is physically located on the control board inside the furnace cabinet. While the fuse itself is inexpensive and replaceable, simply replacing the fuse without finding and fixing the underlying short circuit will result in the new fuse blowing immediately. Diagnosing short circuits in low-voltage wiring requires a multimeter and experience tracing wiring — this is best left to a qualified technician.
Common Causes
When to Call a Professional
Contact a licensed HVAC technician if:
- This code appears — diagnosing and repairing short circuits in 24VAC wiring requires professional equipment and expertise
- The fuse has blown more than once
- You recently had a thermostat, humidifier, or other HVAC accessory installed
- You suspect rodent damage to wiring in your walls or ductwork
- You see visible damage to any thermostat or low-voltage wiring