Dettson furnace inducer starts, but the unit locks out with E4 before ignition; blower may run in cool-down mode.

Safety first: Turn off power at the breaker before removing panels. Components may be hot; allow time to cool.

• 1. Replace or clean air filter: Locate the return air filter (often in the blower compartment or return grille). Replace disposable filters or clean washable ones. Use the MERV rating recommended by Dettson to avoid excessive restriction.
• 2. Check supply and return airflow: Walk through the home and ensure supply registers and return grilles are open and not blocked by furniture, rugs, or drapes.
• 3. Inspect blower motor and wheel: Remove the blower compartment door. Check that the blower wheel is clean and spins freely by hand. Look for excessive dust buildup. Clean the wheel carefully with a brush and vacuum. Verify the motor runs smoothly when power is restored.
• 4. Inspect high limit switch: The high limit is usually mounted on the heat exchanger panel. Check wiring connections for tightness and signs of overheating. Ensure the switch is firmly attached to the surface for proper temperature sensing.
• 5. Reset and test: After addressing airflow issues, restore power and call for heat. Monitor the furnace through a full cycle. If E4 returns quickly, the high limit switch may be weak or the blower speed may be set too low and should be adjusted or replaced by a technician.