Caught out by a Turbomax

Serviced a Turbomax 242/2-5 the other day, ran it at full power in DHW to carry out checks. All was well. Tred out the heating and nothing happened, Status code showing S0 (no demand). Found out that the diverter valve had stuck in DHW mode and the boiler won't demand heat while it's still hot. This one had the old diverter (4mm holes) more prone to sticking than the new one (8mm holes)