I have a Vaillant 83 42 20 combi boiler. This week the hot water has started to run cold. The pressure is fine. There is no fault showing, but in the bottom right of the display panel a C is showing and flashing. I pressed the button that shows a flame with a cross (x) through it, but it switched itself back on.
Have you any suggestions what I can do next?
Based on your description it would appear you are having problems with the combustion process. This could be a number of issues including faulty ignitor, damaged ignitor cable, no gas, low gas or faulty gas valve. Best to call in the expects to deal with this one.