Furnace display shows E20, unit may operate only at low stage or may not modulate correctly (on modulating Thermia furnaces).

Safety first: Turn off power and gas. Gas valve and modulation issues must be handled carefully.

• 1. Inspect modulation wiring: Locate the multi-pin connector between the Thermia control board and the modulating gas valve. Ensure it is fully seated and free of corrosion.
• 2. Check configuration: Verify that any dip switches or parameters related to staging/modulation on the Thermia control board are set according to the installation manual.
• 3. Observe operation: With a call for heat, monitor whether the furnace remains at one firing rate or attempts to modulate. If E20 appears when modulation is commanded, the valve or control may be faulty.
• 4. Technician diagnostics: A qualified technician should measure modulation control signals and gas pressures to determine if the gas valve needs replacement.