how to fix my Vaillant ecotec plus 824?

My boiler is flashing con on display as I have had a rubber pipe split. I have tried hair dryer for 1/2 hour in the electric panel with power off and when I switch it on the boiler works fine but then when the timer switches off and the boiler cools down it starts flashing fault code CON and the boiler does not work full stop (Heating/Hot Water) can anyone advise please how to fix it and what the fault is.

Thanks for your help