How can i fix my Halstead Ace High combi boiler?

I have power to the boiler, the green power light is on, when i turn the heating, hot water or both on, the boiler boiler fires up then after a few few seconds it dies and the green power light flashes,green.
I have got experience in the building game but always get unstuck when it comes to boilers, when they go wrong.
I would really appreciate someones help with this.
If you can advise me what part may have gone, or could it be the PCB,thanks
David