Green FLASH / Red OFF
Open limit switch fault
This code typically points to
- Restricted airflow (dirty filter, undersized duct)
- Blower problem (not running, wrong speed)
- Blower not moving air despite running
- Limit opening under real overtemperature
Next checks to confirm
- Check filter, static pressure, and airflow first
- Verify blower operation and heat speed
- Verify limit recloses after temperature falls
Escalation
Repeated limit opening with normal airflow points toward weak limit, firing-rate issue, or deeper heat-exchanger/airflow problem.
Reset behavior
Clears when limit recloses; repeated failed heat cycles can escalate to lockout.
Urgent
Fix now