istor he325?

hi.

boiler stops working from time to time with very loud vibration when initially firing up, or changing from ch to water. tries 3 times to go over and if cannot, it goes to L and F codes on the display. THis means check incoming gas, which i have, check outside and to the boiler that the condensate is not blocked, done this and it is fine. seen similar posts on the internet mentioning the DV... would love some advice, incming water pressure ok and sealed system running at 1.5 bar.... also, i have not got a clue how to operate the imersion... clues?

thanks a lot. Chris.