Check auxiliary limit control (blower housing)

What is going on

There are two auxiliary limits on the blower housing, one on each side, and they can be manual or automatic reset. Their job is to stop the burners if the main blower fails or if power is lost while the furnace is firing. They work in horizontal right, horizontal left and counterflow installs without being relocated. Before testing, make sure the power is off and the furnace has cooled. A limit that is open should be treated as a symptom, because something made the housing hot, and that cause needs to be found.

Likely causes

• Main blower failure in horizontal or counterflow installs
• Power interruption while the furnace is firing
• Overheating from restricted airflow (open limit)
• Loose or damaged wire on the limit terminals

Check in this order

1. Shut off power at the disconnect and confirm it is off with a meter before touching anything
2. Remove the wires from the auxiliary limit terminals
3. Test continuity across the two terminals with an ohmmeter, only at room temperature, since a hot limit will read open and mislead you
4. If the limit is open, press the red reset button one time only
5. If it opens a second time, find out why before resetting again. Check blower operation, filter and ductwork
6. Repeated resets without finding the cause can damage the heat exchanger, so do not keep pushing the button
7. Reconnect the wires and run a full cycle to confirm normal operation

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)
• Auxiliary limit switch open (blower compartment)