Boiler indicates a communication fault with an external controller (e.g. Bosch room controller or EMS bus).

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Broken or reversed polarity on EMS/bus wiring, Incompatible controller connected, Faulty room controller, PCB communication circuit fault

Safety: Isolate power before working on control wiring.

Steps:

Check the two‑core bus cable between boiler and controller for damage.
Ensure correct polarity and terminals as per Bosch wiring diagram.
Temporarily disconnect the controller and see if the boiler operates in basic mode (if supported).

If communication cannot be restored, the controller or PCB may need replacement by a professional.

Medium 3/5

Room Controller / EMS Bus Module

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