How do you fix a ravenheat csi primary boiler?

I have a ravenheat csi primary boiler which will start the fan, ignite the pilot light then ignite the main burner for less than a second and then cut out. It will then repeat the process over and over again.
I have replaced the ignition board and the gas valve but this has made on change, the overheat thermostat and main thermostat have also been replaced.
I know it would have been cheeper to replaced the whole boiler (it would not have been a ravenheat).
any help would be greatfully received