COMBI BOILER?

HI
THE BOILER FIRES UP IF YOU OPEN A TAP
AFTER 3 SECONDS IT SHUTS DOWN IF YOU LEAVE THE HOT TAPE RUNNING IT WILL TRY AND FIRE UP AGAIN BUT ONLY FOR 3 SECONDS AND IT KEEPS
DOING THIS SAME ON HEATING WHAT PART DOES IT
NEED AND WHERE IS IT LOCATED IN THE BOILER.
THE BOILER IS A RAVEN HEAT HOW MUCH WOULD THE PART
COST BALL PARK FIGURE ONLY THANKS FOR YOUR HELP MOST GRATEFUL TO YOU ALAN