how can i fix my potterton 10/16 boiler?

The boiler was working ok before the gas solinoid was changed now it wont even start, getting no response from it at all. Even the relay has been changed. The gas solinoid was changed because it was passing gas. On the knew relay there is a button underneath which i have pushed up and the relay contacts connect and the boiler fires up for about twenty seconds then goes off. I would appreciate any help thanks. Paul.