EF
Auxiliary switch open
This code typically points to
- Auxiliary limit switch tripped
- Wiring fault on auxiliary switch circuit
- Heat exchanger or cabinet temperature issue triggering auxiliary limit
Next checks to confirm
- Check auxiliary switch continuity and reset if possible
- Inspect wiring on auxiliary limit circuit
- Investigate underlying temperature/airflow cause before clearing
Escalation
Furnace operation inhibited until auxiliary switch closes or fault is corrected
Reset behavior
Clears after auxiliary switch circuit returns to normal.
Safety Critical
Immediate concern