Your boiler's heating power has been manually limited to a lower setting, which might prevent your home from reaching your desired temperature properly.

Maximum heating output: The maximum thermal output can be reduced via the service function 3-b1.

What causes this fault?

This code appears because a settings adjustment has been made to intentionally 'cap' the boiler's maximum power output. It is usually triggered when a technician or user limits the heating capacity to match a smaller property, which can sometimes result in the radiators not getting hot enough during very cold spells. It is not a component failure, but rather a notification that the boiler is not operating at its full potential wattage.