Trusty old Viessmann gave up the ghost yesterday. Flashing fault light. All my heating info is in the same folder which wasn't with the other ones. Typical.
I self installed this boiler in the noughties. Had the same thing when it was near new. Turned out to be the ionization sensor. Eventually bought a new one, which was a different design.
Not being able to find the service manual, I thought I might as well just look at it - only two screws to remove it. And it was a bit dirty.
Gave it a good clean with wire wool and boiler is now running again.
But I've ordered up a new one anyway, at about £40 delivered.
Not quite sure how they work? It appears to be just a metal (of some sort) rod set in a porcelain insulator, and no ground connection.