Don't know - Capacitors are mostly used for smoothing and for timing - in a boiler any large capacitor will probably be part of a time-out circuit for a guess. If your C18 has a + sign on one end indicating that it's polarized then it will be an electrolytic capacitor - these chaps do not last forever - they age more quickly in a warm environment and boliers are warm environments.
A replacement capacitor from an electronics shop will cost pennies - buy the same rating in MicroFarads - and the same or higher voltage rating - the temperature rating may be 85 degrees - 105 degrees will cost a little more if you can get one and it will last longer.
While you are messing with the board it may be wise to replace any other electrolytic caps at the same time.
See this link... on a potterton...
http://kaijaks.co.uk/blog/nick/2008/02/10/dodgy-old-boiler
Good luck...
Peccavi, June 2010
