3 blinks
Limit circuit open
This code typically points to
- Open main high limit due to low airflow or overfired heat exchanger
- Tripped manual-reset rollout
- Blocked flue or heat exchanger
- Poor limit-circuit connection
- Blocked-drain-switch interaction on upflow 90+ can also indirectly produce flame-failure/lockout behavior
Next checks to confirm
- Check filter/airflow restrictions
- Inspect registers/dampers
- Verify temperature rise and firing rate
- Inspect rollout/manual-reset limits
- Inspect flue/heat exchanger
- Inspect limit wiring and continuity
Escalation
Heating sequence interrupted while limit circuit remains open; may participate in later one-hour lockout behavior on newer generations
Reset behavior
Clears when limit circuit is restored; manual-reset rollout must be physically reset; blocked-drain issue requires trap/drain correction rather than blind board reset.
Safety Critical
Immediate concern