Furnace display shows E04 and shuts down; high temperature limit trips during or shortly after burner operation.

Safety first: Turn off power to the furnace. Components may be hot; allow time to cool.

• 1. Check and replace air filter: Locate the return air filter in the Thermia furnace or return grille. If dirty or clogged, replace with the correct size and MERV rating recommended by Thermia. Never run the furnace without a filter.
• 2. Inspect supply and return vents: Ensure that supply registers and return grilles throughout the home are open and not blocked by furniture, rugs, or debris. Restricted airflow can cause overheating.
• 3. Verify blower operation: Set the thermostat fan to "ON". Confirm that the main circulation blower starts and delivers strong airflow from vents. If the blower does not run, runs slowly, or is noisy, inspect the Thermia blower motor, capacitor, and blower wheel for damage or obstruction.
• 4. Inspect high limit switch: Locate the high limit switch on the heat exchanger panel. Check wiring connections for tightness and signs of overheating. With power off, test continuity across the switch when cool; it should be closed. If it remains open when cool, replace the high limit switch with a Thermia-approved part.
• 5. Check blower speed settings (advanced): On some Thermia models, blower speed taps or control board settings can be adjusted. If airflow is low, a technician may need to increase heating blower speed.
• 6. Reset and test: After correcting airflow issues and verifying components, restore power and call for heat. If E04 recurs, have a professional inspect the heat exchanger and duct system for design or sizing issues.