Furnace display shows E18, unit may shut down with a safety alarm related to combustion quality.

Safety first: Turn off power and gas. Combustion faults can lead to carbon monoxide hazards.

• 1. Inspect flue gas sensor: Locate the Thermia flue gas temperature or combustion sensor on the flue outlet or heat exchanger. Check for loose or damaged wiring.
• 2. Test sensor: With the sensor removed, measure resistance at room temperature and compare to Thermia specifications. Replace if out of range.
• 3. Check for soot or corrosion: Excessive soot in the flue or on the heat exchanger indicates poor combustion. Do not operate the furnace until a technician has inspected and cleaned the combustion system.
• 4. Professional combustion analysis: A licensed technician should perform combustion testing and adjust gas pressure and air settings as needed.