Furnace display shows E12, unit may not respond to thermostat changes or may cycle erratically.

Safety first: Turn off power before working on low-voltage wiring.

• 1. Verify thermostat compatibility: Ensure the thermostat is suitable for a forced-air gas furnace and wired according to Thermia and thermostat manufacturer instructions.
• 2. Inspect thermostat cable: Check for damage where the cable enters the wall and furnace. Look for staples through the cable or rodent damage.
• 3. Bypass thermostat: At the Thermia control board, remove the thermostat wires and install a jumper between R and W. Restore power. If the furnace operates normally without E12, the thermostat or cable is likely faulty.
• 4. Check thermostat settings: Confirm that the thermostat is set to HEAT mode and that any advanced settings (system type, cycles per hour) are correct.
• 5. Replace thermostat if needed: If wiring is good but E12 persists only when the thermostat is connected, install a new compatible thermostat.