4 red flashes
Main limit switch opened during call for heat
This code typically points to
- Dirty filter
- Undersized or restricted duct system
- Wrong blower speed
- Loose limit wiring
- Faulty blower motor
Next checks to confirm
- Check filter, static pressure, airflow first
- Verify blower speed and motor operation
- Verify firing rate and limit circuit
Escalation
If limit does not re-close in time, escalates to 11 red. Repeated trips five times trigger 1-hour soft lockout.
Reset behavior
1-hour soft lockout after repeated trips; otherwise clears when limit recloses.
Urgent
Fix now