4
Limit or flame rollout switch open, or furnace forced into high-heat-only mode due to two successive low-heat limit trips
This code typically points to
- Loose blower wheel
- Defective switch/connections
- Improper low/high gas input adjustment
- Improper limit switch/no gasket
- Dirty filter
- Restricted duct system
Next checks to confirm
- Check whether it is actual open limit/rollout versus high-heat-only fallback
- Then check airflow and gas input
Escalation
Rollout requires manual reset and combustion review.
Reset behavior
If open >3 minutes changes to code 7; if shorter, code 4 continues until blower shuts off.
Safety Critical
Immediate concern