Control panel shows Er4 and furnace shuts down or will not start a new cycle.

Safety first: Turn off and unplug the furnace. Allow it to cool before accessing snap discs or wiring.

Steps:

• 1. Verify fuel and ignition: Make sure the hopper has fuel and the igniter is working. Weak ignition can prevent the low limit from reaching temperature.
• 2. Locate the low limit switch: Often mounted on the exhaust housing or near the heat exchanger, labeled as "Low Limit" or "Proof of Fire" with two wires attached.
• 3. Inspect wiring: Check for loose spade terminals, burnt connectors, or damaged insulation. Reseat connectors firmly.
• 4. Test the switch: With the furnace cool and power off, disconnect one wire and check continuity across the switch. Many low limit switches are normally open and close when hot; consult your manual. If the switch never closes during a full burn cycle (test with a meter if possible), it may be defective.
• 5. Clean burn pot and exhaust: Poor combustion from a dirty burn pot or plugged exhaust can keep temperatures too low. Remove and clean the burn pot, air holes, and ash traps. Clean the vent pipe and exhaust blower housing.
• 6. Replace the low limit switch if faulty: Install the correct temperature-rated US Stove low limit snap disc. Do not substitute with a different temperature rating.
• 7. If Er4 persists: After confirming good combustion and a good switch, the control board may not be reading the signal correctly. Have a technician evaluate the board.