OFF
Secondary voltage fuse is open
This code typically points to
- Short circuit in secondary-voltage (24VAC) wiring
Next checks to confirm
- Inspect 24VAC circuit for shorts before replacing fuse
- Replace fuse and disconnect thermostat leads to isolate short circuit
Escalation
Escalate only after isolating field low-voltage wiring and accessories if fuse continues to open.
Reset behavior
Restores when short is corrected and fuse is replaced.
Urgent
Fix now