Medium Lockout
bF on Viessmann Vitodens 333

Your boiler's internal communication system has detected an issue with a plug-in connection module, meaning the main control board cannot talk to other system components properly.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Control mode - Incorrect LON communication module

What causes this fault?

This fault occurs when the boiler's internal computer fails to recognize or communicate with the plug-in communication module. The most common reason is that the module has either worked itself loose, been installed incorrectly, or has suffered an electrical failure that prevents it from sending data to the main control board.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

b7View →b8View →bcView →

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