Baxi Combi HE 133 Plus dry fire problem - can anyone help?

My Baxi Combi HE 133 Plus boiler (3/4 yrs old) keeps showing the dry fire fault lights. A registered engineer has looked at it and can't find any problems, and if I reset the system it ticks over ok for a few weeks before showing the fault again. One of our radiators needs bleeding a lot (almost every 1/2 weeks) - but I'm not sure if these two things are connected. Any thoughts?