You start by finding out what the likely cause is.
The simplest way is to do a Google search for - say - "ariston combi error 108"
There should be quite a few results that will point you in the right direction. I saw one from aristonboilersguide.co.uk that gave a nice set of detaile din structions and another from a Screwfix forum that even had a couple of photos
Anon, January 2017
