Auxiliary limit switch open (blower compartment)

What is going on

This one means the auxiliary limit in the blower compartment is open, and the furnace will not run until it closes. It is a safety, so it is telling you something got too hot or lost power at the wrong moment. Think about airflow first, since a clogged filter or a slow blower is the most common reason, and then the wiring. If the 115 V supply was shut off in the middle of a heat cycle, the limit can trip from residual heat, and that is a one-off rather than a fault.

Likely causes

• 115 V power supply shut off during heat cycle
• Not enough conditioned air over the heat exchanger from blocked filters, restrictive ductwork, improper circulator blower speed or failed motor
• Loose or improperly connected wiring in the high limit circuit
• Closed dampers or registers cutting airflow

Check in this order

1. Turn power off before repair and let the cabinet cool
2. Check filters and ductwork for blockage. Clean filters or remove obstructions
3. Check circulator blower speed and performance. Replace motor if needed
4. Tighten or correct the wiring connection at the limit
5. Check continuity across the limit with the wires off, only when cold
6. Restore power and run a full cycle, watching that the blower and burners behave normally
7. If it trips again, find the cause before resetting, and call a licensed tech if you cannot

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Blower motor does not have enough information, fails to start 40 consecutive times