Printed Circuit Board
PCB's are the catalysts between the other boiler components - they feed it information and it acts accordingly. There are lots of different components on a PCB such as relays, rectifiers and transformers. Many boards are replaced unnecessarily because it's assumed that because they look so complicated then whatever the fault is it must lie within the board. If a board has obvious signs of distress, burn or scorch marks this may be the case however if not test all the components individually connected to the board, unplug and replace all the harness connections to ensure good contacts before deciding to replace a circuit board as they are not only very expensive themselves but if the problem is not the board you are not going to solve the problem.