E12 displayed; controller indicates room thermostat communication or sensor error

Safety: Turn off power before working on low‑voltage thermostat wiring.

• Locate the room thermostat or room sensor connected to the Defro controller terminals.
• Check that the wires are firmly clamped and not shorted together.
• Inspect the cable along its route for damage from nails, rodents, or pinching.
• If using an external thermostat, confirm it is configured correctly in the Defro controller menu (on/off vs. OpenTherm or bus type, depending on model).
• Temporarily bridge the thermostat terminals (if allowed by the manual) to see if the furnace responds; if it does, the thermostat or cable is faulty.

Replace the thermostat or room sensor if confirmed defective. For bus‑type controls, a technician may be needed to diagnose communication issues.