Bryant 355MAV Error Code 24: Secondary Voltage Fuse Is Open
What Does Code 24 Mean?
Status code 24 on the Bryant 355MAV indicates that the secondary voltage (24 VAC) fuse on the control board has blown. This fuse protects the control board's low-voltage circuitry from damage caused by short circuits in the thermostat wiring or other 24-volt components connected to the system.
The most common cause is a short circuit in the thermostat wiring. This happens when two thermostat wires touch each other — either inside the wall where the wiring runs, at the thermostat terminal block, or at the furnace control board connections. Damaged or pinched wires during renovations, rodent damage to wire insulation, or an improperly installed thermostat can all create shorts.
When the 24V fuse blows, the furnace loses all control-level communication. The thermostat cannot send signals to the furnace, and the control board cannot operate the gas valve, inducer, or other 24V components. A technician needs to first find and repair the short circuit in the wiring, then replace the fuse (typically a 3-amp automotive-style fuse on the control board). Simply replacing the fuse without fixing the short will cause it to blow again immediately.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Short circuit in 24 VAC thermostat wiring | Most common | ✗ Call a pro → |
| Damaged or pinched thermostat wire | Common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- The furnace shows code 24 — a technician must locate and repair the short circuit before replacing the blown fuse
- The thermostat has no power or the furnace does not respond to any thermostat commands
✓ Verified against manufacturer service manual — March 2026