baxi 100 he system fault?

boiler reaches temperature but but pump stops after aprox 2 minutes will not come back on as no water circulation through primary flow/return which is up to temp , will reignite if turn off prog then back on , same thing hapens every time 3 way valve working o.k have changed programmer,cylinder stat working correctly is it pc board or limit stat ,as sometimes if left lockout light has to be reset