Vaillant Turbomax Plus- Hot Water Problem

[DanPlumb]

Found a problem with divertor valve on this vaillant, replaced divertor as it was stuck in heating position and heat was directed to radiators rather than hot water. The boiler then fired for hot water how ever the boiler would only reach 40 degrees, i contacted vaillant who advised to replace the aquasensor, I done this this evening, improved temperature but still runs hot/cold. Now thinking heat exchanger blocked. Anybody else came across same fault or is there another part that coud be playing up?

Thanks