Carrier 58MVP Error Code 24: Secondary Voltage Fuse Is Open
What Does Code 24 Mean?
Status code 24 on the Carrier 58MVP means the secondary voltage fuse is open. This fuse protects the 24-volt circuit that powers the thermostat, transformer secondary, and other low-voltage accessories connected to the furnace.
A blown 24V fuse is almost always caused by a short circuit in the thermostat wiring. Common scenarios include bare thermostat wires touching each other inside the wall, a wire being pinched between the thermostat base plate and the wall, or damaged insulation on wires running through the house. If a smart thermostat was recently installed, incorrect wiring connections are a frequent cause.
Replacing the fuse alone will not fix the problem — it will blow again as soon as the short is reintroduced. A technician needs to disconnect the thermostat wiring at the furnace and test each wire for shorts to ground and to other wires. Once the short is located and repaired, the fuse can be replaced and the system restored.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Short circuit in 24 VAC thermostat wiring | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- When this code appears — tracing and repairing a short circuit in thermostat wiring requires professional diagnostic tools and expertise
✓ Verified against manufacturer service manual — March 2026