However, there are two things you can try before calling the pros:

Tools needed: Torx T20, fine sandpaper or steel wool, multimeter with microamps.

The indicates a communication or external reporting fault typically involving the EA1 extension module or an external safety device. While it implies an external component issue rather than a core boiler failure, resolving it requires a systematic approach.

Sometimes the fault is a temporary logic glitch.

Look up the specific for your model (e.g., Vitodens 100 or 200) Explain the reset procedure for your specific control panel Just let me know your boiler model ! Viessmann Vitodens 200-W and 300-W fault codes