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Viessmann fault code

Viessmann A9: communication error, OpenTherm device

Engineer only

Viessmann describes A9 on Vitodens 100-W (B1HC/B1KC) boilers as: communication error, OpenTherm device.

What you can safely do

There is nothing you can safely do yourself with this one. Leave the boiler off and book an engineer.

Find a Gas Safe engineer near you Opens the official Gas Safe Register search.

Applies to: Vitodens 100-W (B1HC/B1KC). Codes can differ between models, so check your manual.

From Viessmann's own fault tables. Your model name is usually on the front of the boiler or in the user manual.

Model	Meaning
Vitodens 100-W (B1HC/B1KC)	Communication error, OpenTherm device

Can I fix Viessmann A9 myself?

No. It needs a Gas Safe registered engineer to diagnose. Tell them the exact code and your boiler's model name.