It's an overheat. Lots of stuff can cause this, not necessarily a pcb.
If you are sure the pump is running strongly (remove pump cap and try to stop it with a screwdriver - yes there will be water!), make sure your flow pipe is getting hot - i.e. make sure the heat is leaving the boiler and you aren't getting any kind of restriction in your ch system. You are looking for around an 11 degree differential between flow and return.
Worth checking out that primary sensor too, can do this if you have a multimeter. Also check overheat stat has not failed.
Please be careful - if you decide to check the pump, cover the pcb box with a towel, and be aware that there is 240v to that pump.
BunSlinger
September 2008
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.