It is a known fault that the diverter valve can stick/fail. I would expect the boiler to fire up from cold and heat itself up for a few seconds if the heating is turned on.
You dont say when it was fitted but could be within a five year warrantee.
I have not experienced that failure so dont know if its possible to dismantle and clean the diverter.
Look to see if the tap symbol is there all the time. If it is, the domestic flowswitch is stuck on, usually caused by debris in the water. Try turning the tap thermostat knob fully anticlockwise; if you then can get central heating, that confirms the diagnosis.
The diverter valve on the Viessmann boiler has been very problematic. It is however very simple to replace. It is worth contacting viessmann to ask for this to be fixed under warranty. If you warranty is over ask anyway as this is a known fault. The replacement parts are reliable. Dirt in the flow switch is easily removed. This boiler is the most simple boiler ever made to repair and service.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local