24
Secondary voltage fuse open
This code typically points to
- Short in 24V thermostat/accessory wiring
- Shorted outdoor-contact circuit
- Shorted humidifier/accessory
- Board fuse blown during service or install
Next checks to confirm
- Isolate low-voltage wiring
- Inspect thermostat, outdoor, and accessory circuits
- Replace fuse only after fault isolation
Escalation
Repeated fuse failures after field wiring isolation raise suspicion of board/transformer issue.
Reset behavior
Clears after fuse replacement and fault correction.
Urgent
Fix now