Hi everyone. About a week ago, our Vaillant 937 began to display fault codes F81/F91. The temperature on the display would rise rapidly from zero to 80 in a couple of seconds thereby shutting down the boiler. initially, this was on the heating side because with it turned off the hot water worked happily. Now the symptom has spread to both sides and nothing works. The boiler fires just fine but within seconds cuts out because of the rapid rise in temp. Any ideas.
Many thanks Lynne
Many thanks Lynne