4 red flashes
High-limit switch open, defective temperature sensor, or open 24V fuse
This code typically points to
- Dirty filter
- Duct restriction
- Blower problem
- Open board fuse
Next checks to confirm
- Determine which branch is actually open: limit, sensor, or fuse
- Verify airflow and blower
- Inspect 24V fuse
Escalation
Multiple possible causes; isolate before replacing components.
Reset behavior
Clears when condition is resolved.
Urgent
Fix now