alpha cd24c?

hi, my alpha cd24c is locking out on failue to light solid B neon error, only when the panel is on, when the panel is off it works as should, and for a time when panel is reassembled whilst boiler is running, I can only assume as this is not my field in gas, that there is a lack of air for the lighting sequence to sufficiently take place, although pressure hoses + & - are ok, and the fan is running, flue is clear also, does anybody know of a air sensor of sorts could be the problem or has had this problem before? no pressure switch in this boiler. + hose direct to gas valve.