Boiler shows E12, no response to thermostat or control signals.

Safety: Isolate power before working on control wiring.

• Check boiler local controls: Try setting the boiler to manual or constant mode (if available). If it runs in manual but not via the room thermostat, the issue is with external controls.
• Inspect thermostat wiring: Check the cable from the room thermostat or programmer to the boiler for damage, loose connections, or water ingress.
• Verify wiring terminals: Ensure the thermostat is connected to the correct Thermona low-voltage control terminals (e.g., eBUS/OpenTherm) as per the installation manual.
• Test with link (engineer level): Temporarily fit a link wire across the boiler's room thermostat terminals to simulate a call for heat. If the boiler runs, the external control or wiring is faulty.
• Controller compatibility: Confirm that any smart thermostat used is compatible with Thermona boilers and configured correctly.