Burners ignite but shut off within a few seconds, furnace retries and then locks out with E03.

Safety first: Turn off power and gas before servicing. Allow burners to cool.

• 1. Clean the flame sensor: Remove the flame sensor from the burner assembly. Gently clean the metal rod with fine steel wool or an abrasive pad until shiny. Do not sand the porcelain insulator. Reinstall and ensure it is fully in the flame path.
• 2. Check ground connections: Verify that the burner rack and furnace chassis have solid ground connections to the Thermia control board. Tighten any loose screws and clean rusted contact points.
• 3. Inspect burner flame: Restore power and gas, then observe the burner flame through the sight glass. It should be steady and mostly blue. Yellow, lifting, or unstable flames can reduce flame sensor signal—have a technician check gas pressure and burner orifices.
• 4. Measure flame sensor current (advanced): With a microamp meter in series with the flame sensor lead, measure flame current during burner operation. Typical acceptable values are 1–5 µA DC (consult Thermia specs). If low despite a clean sensor and good flame, the ignition control module may be faulty.
• 5. Inspect wiring to control board: Check the flame sensor wire for cracks, burns, or loose connections at the Thermia main control board. Replace damaged wiring.
• 6. If E03 persists: The ignition control module or integrated furnace control board may need replacement. This should be done by a qualified technician familiar with Thermia furnaces.