How can I fix my Vaillant Turbo Max plus Combo boiler?

Recently my hot water stopped working after a long expensive time the engineer finally found it was the aqua sensor which he replaced.
However now while the hot water works the central heating doesn't, all it does is heat a couple of radiators while all the others are cold.
I have bled the radiators but they had no air in them, is there a way that there is air somewhere else in the system that is stopping the heating system from working