Furnace displays Er3 and stops feeding fuel; combustion blower may continue to run.

Safety first: Turn the furnace OFF, disconnect power, and allow the unit to cool completely. Do not bypass safety switches.

Steps:

• 1. Let the unit cool: Wait at least 30–45 minutes so the high limit switch can reset if it is an automatic-reset type.
• 2. Check air filters and return air: Dirty or blocked filters and closed registers can cause overheating. Clean or replace filters and ensure adequate return air to the furnace.
• 3. Inspect the convection blower: Verify that the room/heat distribution blower runs when the furnace is operating. If it is noisy, slow, or not running, check for obstructions, clean the blower wheel, and test the motor capacitor (if equipped).
• 4. Locate the high limit switch: Typically mounted on the heat exchanger or hot air plenum with two wires attached. Inspect for loose connections or signs of overheating.
• 5. Test the high limit switch: With power off, disconnect one wire and check continuity across the switch at room temperature. It should read closed (near 0 ohms). If it remains open after cooling, replace the switch with the correct US Stove high limit switch.
• 6. Clean internal passages: Excess ash or soot in the heat exchanger or internal passages can cause hot spots. Follow the manual to open clean-out covers and vacuum ash with a metal ash vacuum.
• 7. Restart and monitor: Restore power, start the furnace, and monitor operation. If Er3 recurs quickly, stop using the unit and contact a qualified technician to check for undersized ductwork, blower issues, or incorrect installation.