Valliant Eco Tech 615 boiler ?

since swapping a rad and bleeding boiler wont start in morning showing an F.15 code.( PCB board was replaced in June 2019 ) engineer at a loss and Valliant say replace PCB.( again) boiler is 9 years old and board are not cheap. can manage to restart boiler by switching power off/on a few times. it WILL run 24/7 if we leave thermostat on 22degrees? engineer can't say why it works at 22 and over and not under.