wickes heat 40 aps fault?

friends boiler seems to be working in every other respect, but, when he turns it on it goes straight into lockout. he then turns it off at the mains and turns it back on, removes one terminal from the aps, the fan starts up, replaces the connector again, the boiler then works fine until the internal thermostat is satisfied, then the whole thing starts over again.
anyone got any suggestions please. ty in anticipation

ty for the reply kev, but test what? and what should i see?
as the guy aint got a manual and although i have looked for him, i am having trouble getting one.

My advice is that as the fan and APS are combustion related you and your friend should call a registered gas engineer!

Tony