Boiler starting and stopping

[ohdaesu]

Hi all, we have a biasi riva m96a.28smc combi which has started running odd, it lights for anything up to 30 secs and stops for a similar time continually, when hot water is called for or when the central heating is on.
I have scoured the Internet for answers and have replaced the secondary heat exchanger but this has made no difference.
I would be very grateful if anyone has any other suggestions.

 

[Gasguru]

It may be a temperature sensor fault but more likely an issue with the air flow and diy advice regarding the combustion chamber is not permitted here.