problem dry fire baxi 133he plus?

i own a baxi 133he plus which has worked fine all day then all of a sudden when i turned the heating up it went into dry fire mode red light on and two green lights flashing underneath.pressure spot on no air in system.boiler broke down before xmas and a baxi engineer replaced diverter valve and mains pcb so it cant be the pcb what else could it be.

lee lewis
January 2013