Carrier 58MVP Error Code 24: Secondary Voltage Fuse Is Open
What Does Code 24 Mean?
Status code 24 on the Carrier 58MVP indicates the secondary-voltage fuse is open. The control board has a low-voltage (24 VAC) fuse that protects the transformer secondary; when it blows, the board loses control power on that circuit. Carrier's guide points to a short circuit in the 24 VAC wiring as the cause.
The most common source is a short in the thermostat wiring — a pinched or chafed wire, a staple driven through a cable, or a shorted accessory such as a humidifier or zone control on the 24-volt circuit. On this control the fuse is a protective device, so simply replacing it without finding the short will just blow the new fuse.
Locating a wiring short and safely replacing the board fuse is electrical diagnostic work for a technician, not a homeowner repair.
What You'll Notice
- The furnace and thermostat may be unresponsive on the low-voltage circuit
- The board LED flashes code 24, or the board appears unpowered on 24 V
- The problem often follows recent thermostat, humidifier, or zoning work
- A replaced fuse blows again quickly
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Short circuit in 24 VAC thermostat wiring | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician disconnects the thermostat and accessory wiring to isolate the short, checks the R and C circuit for a direct short to ground, and inspects cable runs for pinched or chafed insulation. Once the fault is found and corrected, the correct-rated fuse is installed. This is low-voltage electrical diagnosis performed by a professional.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- The board shows code 24 or the low-voltage circuit is dead
- A replacement fuse blows again shortly after installation
- The problem started after thermostat, humidifier, or zone-control wiring changes
Frequently Asked Questions
Can I just replace the blown fuse on my Carrier 58MVP?
Replacing the fuse without finding the short will usually just blow the new one. A technician needs to locate and fix the wiring short first.
What causes code 24?
A short circuit in the 24-volt wiring, most often the thermostat cable or a shorted accessory on that circuit.
Sources
✓ Verified against manufacturer service manual — March 2026