Furnace display shows E13, blower runs but burners do not light, often after a power outage or surge.

Safety first: Turn off power at the breaker before touching the control board.

• 1. Power reset: Turn off the furnace breaker for at least 5 minutes, then restore power. Some transient faults clear after a full reset.
• 2. Inspect control board: Look for burn marks, cracked solder joints, or swollen capacitors on the Thermia main control board. Check that all plug-in connectors are fully seated.
• 3. Verify proper grounding: Ensure the furnace cabinet is properly grounded and that the ground wire to the control board is secure. Poor grounding can cause erratic board behavior.
• 4. Surge protection: If E13 occurred after a storm or outage, consider installing a surge protector on the furnace circuit to prevent future damage.
• 5. Replace control board: If E13 persists and other components test good, the control board is likely defective and should be replaced with the exact Thermia replacement by a qualified technician.