My 17 month boiler experienced a problem.....the outside temp sensor stopped working....so the controller went into a default mode and assumed the outside temp was 0c....thus meant the heating still worked but was making the boiler work harder than it needed to.
Call to Vaillant on Fri lunchtime.....explained the problem....heating type blokey booked for today!
Turned up about 9.30am, replaced everything to do with the controller....that is controller + circuit board for controller fitted in the boiler + the outside temp sensor....
Carnt fault that service.....fixed within 24hrs of call under warranty
I did ask why he was changing the whole lot and not just the outside temp sensor.....apparently it comes as a complete kit....so might as well change the lot