Furnace display shows E08, blower may run continuously without heat, or system may not respond to thermostat.

Safety first: Turn off power at the breaker before inspecting electrical components.

• 1. Check breaker and service switch: Verify that the furnace breaker is ON and that any local service switch near the Thermia furnace is also ON.
• 2. Inspect control board fuse: Remove the access panel and locate the low-voltage automotive-style fuse (typically 3–5A) on the Thermia control board. If blown, inspect thermostat and accessory wiring for shorts before replacing with the same rating.
• 3. Test transformer output: With power on and using a multimeter (only if you are comfortable and qualified), measure AC voltage across the secondary (R and C terminals). It should be around 24 VAC. If there is no output but primary power is present, replace the transformer with a Thermia-approved unit.
• 4. Inspect low-voltage wiring: Look for bare wires touching the furnace cabinet or each other, especially near the thermostat, humidifier, or outdoor unit connections. Repair or replace damaged cable.
• 5. After corrections: Replace the fuse if needed, restore power, and test furnace operation. If E08 recurs, the control board may have internal power regulation issues and require replacement.