ravenheat csi primary boiler fault?

after having had the boiler off for system mods and refilling and direct wiring the pump to circulate any air and re bleeding, when restarting the boiler the pump comes on fan on ign on and boiler fires and after approx 10 secs a thumping noise in the pipes at the boiler is heared then boiler shuts down.have replaced fan control board as similar fault was cured 3 years ago with this and tested pressure switch which clicks when a gentle suction is applied to the clear tube.also fan is spinning free and have wd40 the shaft
any thoughts on solution thanks