off
Internal control failure / no control signal
This code typically points to
- No 115 V power to furnace
- No 24 V power to integrated control
- Blown fuse or breaker
- Internal control fault
Next checks to confirm
- Check 115 V power to furnace
- Check 24 V low-voltage power at control
- Inspect control fuse
- Inspect breaker/disconnect
- Replace control only after power/fuse checks are verified
Escalation
Control enters locked-out state if internal fault is confirmed
Reset behavior
Restore power/fuse condition if external power loss caused the state; replace control if internal fault is confirmed.
Urgent
Fix now