This part is located within the printed circuit board section. The printed circuit board or ( pcb ) is the boilers central control. It constantly checks every process that occurs via the thermostats, switches and sensors . It also enforces the safety procedures, making sure that the boiler ceases to function if any of the internal readings point to any kind of error or fault. The printed circuit board is effectively the brain of your boiler, It is programmed to ensure everything is running as it should .