I have a Vaillant TURBOmax Plus. Both the HW and CH are intermittently cutting out with error S.33, check air/flue system. I powered it up and watched the air pressure switch signal, D.24, it was not changing state, i.e. D.24 = 0. I can hear the fan going and the fact that the problem is intermittent implies that the flue is not blocked. I was going to test the air switch electrically, by removing it, but then I noticed that the boiler sometimes cuts out with error S.07, pump over run. Might these problems be related? Would somebody explain pump over run to me, please.