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 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 circuit 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, or pinched — can also cause this fault.
Replacing the fuse without finding and repairing the short will blow the new fuse immediately. A technician will disconnect thermostat leads at the control board terminal strip first to isolate whether the short is in the thermostat wiring or in other 24VAC components, then trace the wiring systematically before installing a replacement fuse.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Short circuit in 24VAC wiring | Most common | ✗ Call a pro → |
| Defective thermostat wiring | Common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- Call a technician to locate and repair the short circuit before replacing the fuse; replacing without finding the short causes immediate re-failure
- If the new fuse blows immediately after replacement, the short has not been found — the full 24VAC wiring needs to be traced
✓ Verified against manufacturer service manual — March 2026