Biasi Riva m96 locking out

[Purlaboy]

Ok, I'll try to be comprehensive here:
My Biasi boiler developed a fault where it was buzzing on start up but not firing up, the buzzing noise was from the ignition PCB which I replaced.
Now, it fires up straight away but locks out after a few seconds. The only way I can get it to come on and stay on is if I continually hit the reset button around 10 times in succession, then eventually it stops locking out and works just fine.
So my guess is maybe a faulty sensor.
Could anyone shed any light on this? I'd like to narrow it down if I can.
Thanks
Steve

 

[Ihavenojob]

The ignition control pcb controls several things, including gas pressures (which most likely will be the fault) and would need setting up with equipment that the DIYer wouldn't usually have. I would suggest getting a RGI in to check it out.
Is it OK for hot water, as I think it fires at a higher rate for hot water so it senses the flame better than for heating, which will fire at a lower rate.

 

[Purlaboy]

It fires up straight away and with full flame and runs for several seconds, enough to heat the rads if I keep resetting as soon as it cuts out which is why I thought it could be a sensor. There doesn't seem to be a problem with the gas flow as the burners burn fully when I switch it on as they do when it stays running after several attempts at restarting.