Bryant 315AAV Error Code 24: Secondary Voltage Fuse Is Open
What Does Code 24 Mean?
Status code 24 on the Bryant 315AAV signals that the secondary voltage fuse — the 3-amp fuse on the control board that protects the 24-volt circuit — has opened. This fuse blows to protect the board and transformer when the low-voltage wiring shorts, typically at the thermostat wiring or an accessory connection.
Unlike a continuously OFF LED (no power at all), with code 24 the board still has line power and can flash the code, but the 24-volt loads it drives — the gas valve, relays, and controls — have lost their protected supply, so the furnace cannot run. Bryant's data lists a short circuit in the secondary voltage wiring as the cause; in practice that often means damaged, pinched, or staple-nicked thermostat wires, or two conductors touching at the thermostat or furnace terminals.
Simply replacing the fuse without finding the short will just blow the new fuse. Locating a low-voltage short and replacing the board fuse is a technician task involving meters and wire tracing, not a homeowner repair.
What You'll Notice
- The furnace is unresponsive to the thermostat even though the control still shows a code
- The status LED flashes two short then four long flashes
- A thermostat powered from the furnace may go blank because the 24-volt supply is cut
- The problem may follow recent thermostat replacement, wiring changes, or rodent or construction damage to low-voltage wires
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Short circuit in 24V thermostat wiring | Most common | ✗ Call a pro → |
| Damaged or pinched thermostat wires | Common | ✗ Call a pro → |
How This Is Diagnosed
A technician isolates the short before replacing the fuse: they disconnect the thermostat and accessory wiring and check whether the fault clears, then inspect the thermostat cable and terminals for pinched, nicked, or touching conductors. Once the short is found and corrected, they replace the 3-amp secondary fuse on the control board and confirm the 24-volt circuit holds under load.
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 furnace is dead to the thermostat but the board still flashes code 24
- A replaced fuse blows again, indicating the short is still present
- The problem started after thermostat or low-voltage wiring work
- Any low-voltage wiring diagnosis or board-fuse replacement, which is not a homeowner repair
Frequently Asked Questions
Can I just replace the blown fuse on my Bryant 315AAV?
Replacing the 3-amp fuse without finding the short will only blow the new fuse. A technician needs to locate and fix the low-voltage short first, then replace the fuse.
What usually causes the 24-volt fuse to blow?
A short in the low-voltage wiring — commonly damaged or pinched thermostat wires, or two conductors touching at a terminal. Bryant lists a short in the secondary voltage wiring as the cause.
Is code 24 the same as the board being dead?
No. With code 24 the board still has line power and can flash the code; only the protected 24-volt supply is cut. A completely dark LED instead means the board has no power at all.
Sources
✓ Verified against manufacturer service manual — March 2026