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Hi, I have a Worcester greenstar 28i Junior combi mk111, that works fine in CH mode but locks out in DWH after 1-2 mins, Slow Flashing blue (light mostly on) with flashing reset showing, I have re pressurised the expansion vessel as it was flat and Pressure is reading about 1.2bar now and holding. The fan was pulling minus 1.2mbar, so cleaned out the hex including the burner and it is now pulling a healthy minus 7.2mbar, there wasn't much left of the electrodes when I took it out to clean the hex. So I fit new ones along with a new seal of course. But alas still locks out under DHW demand. changed the high limit stat still no joy. No water leaks. wiring harness and connections are good and pump runs good with no unusual noises and the flue is clear. So I bit the bullet and changed the control board... Guess what... still locks out when hot tap running. So if anyone has any suggestions I would love to hear them. (me thinks maybe plate exchanger now)??? after that I,m stumped.