Bryant 340MAV Error Code 24: Secondary Voltage Fuse Is Open
What Does Code 24 Mean?
Code 24 on the Bryant 340MAV indicates the secondary voltage fuse on the Carrier integrated furnace control board has opened (blown). The fuse protects the 24VAC transformer output — when it blows, the control board, thermostat, and all 24VAC accessories lose power and the furnace shuts down completely.
A blown secondary fuse is caused by a short circuit in the 24VAC wiring. The most common source is a short in the 24VAC thermostat wiring, where insulation damage creates a direct path between the R (hot) and C (common) terminals. Defective thermostat wiring — wires that have been cut, chafed, pinched, or stapled through — can also cause this fault.
Replacing the fuse without finding and repairing the short will blow the new fuse immediately. Note that a blown 24VAC fuse (code 24) will produce a dark board similar to code-off, but the two have different roots: code-off usually means no power is reaching the board at all, while code 24 specifically records that the board's own protective fuse opened because of a downstream short.
What You'll Notice
- The furnace is completely dead, and the thermostat display may be blank because it lost its 24VAC common
- The control board LED is off because the 24VAC fuse has cut power to the low-voltage circuit
- A replacement fuse blows again immediately, confirming a live short
- The failure often appears after thermostat work, new accessory wiring, or a pinched or chafed wire
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Short circuit in 24VAC wiring | Most common | ✗ Call a pro → |
| Defective thermostat wiring | Common | ✗ Call a pro → |
How This Is Diagnosed
A technician isolates the short by disconnecting the thermostat leads at the board terminal strip and testing whether a new fuse holds — if it does, the short is downstream in the thermostat wiring; if it still blows, the short is in the furnace-side 24VAC components or wiring. The suspect circuit is then traced for pinched, chafed, or staple-damaged wire before a same-rating fuse is installed.
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:
- A new fuse blows immediately after replacement, meaning the short has not been found
- The failure followed thermostat, zoning, or accessory wiring work
- Low-voltage wiring needs to be traced for a pinched or chafed short
- The furnace and thermostat are both completely unpowered
Frequently Asked Questions
Can I just replace the blown fuse on my furnace board?
Replacing it without finding the short will blow the new fuse instantly. The short in the 24VAC wiring must be located and repaired first, which requires a technician.
What causes the 24-volt fuse to blow?
A dead short between the hot (R) and common (C) low-voltage wires — commonly damaged thermostat wire, a pinched cable, or a miswired accessory. It is an electrical fault, not a homeowner repair.
✓ Verified against manufacturer service manual — March 2026