alpha 280e combi

[nige1]

Hi
The boiler will run for a few seconds then trip the red reset button but will only do this on heating if I switch to water only the boiler will not even fire up
any ideas
thanks

 

[wnjswozo]

high limit stat,check pump,fan ,air pressure switch

 

[nige1]

Hi checked all replaced 2 pcb boards electrode still no joy