Boiler displays F90, burner does not start, communication error indicated

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

Faulty communication cable between main PCB and burner control, incorrect module fitted, internal PCB fault

Safety: Isolate power before accessing internal modules.

Check module seating: Ensure any plug-in burner or expansion modules are fully seated in their slots.
Inspect comms cable: Look for damage to any ribbon or data cables linking boards.
Part compatibility: Verify all modules are approved for the specific Viessmann boiler model.
Module replacement: Replace the faulty module or main PCB as indicated by diagnostics, then reconfigure parameters.

Hard 4/5

Internal Communication Cable / Expansion Module

Check Price on Amazon

We may earn a commission from links on this page.