Furnace display shows E06, unit will not start, control board LEDs may indicate a communication error.

Safety first: Turn off power to the furnace before working on wiring or control boards.

• 1. Inspect thermostat wiring: Remove the furnace access panel and locate the low-voltage terminal strip (R, C, W, Y, G) on the Thermia control board. Ensure all wires are firmly attached and not shorted together. At the thermostat, verify that wires are connected to the correct terminals per Thermia and thermostat instructions.
• 2. Bypass thermostat (test): With power off, temporarily connect a jumper between R and W at the furnace control board. Restore power. If the furnace starts normally without E06, the thermostat or thermostat cable may be faulty.
• 3. Check for external controls: If your Thermia furnace is connected to a zoning panel, humidifier, or other accessory, disconnect these one at a time to see if E06 clears. A shorted accessory can disrupt communication.
• 4. Inspect control board: Look for burn marks, swollen components, or corrosion on the Thermia main control board. Ensure all multi-pin connectors are fully seated.
• 5. Power cycle: Turn off power for 5 minutes, then restore. Some communication faults clear after a full reset.
• 6. If E06 remains: The main control board may be defective and should be replaced with the exact Thermia replacement board by a qualified technician.