worcester 28Si II hot water fault?

My Worcester boiler heats the central heating water ok, and seems to heat the hot tap water if the central heating off. But if the central heating is on and hot water is drawn the boiler faults out and displays the rapid flashing fault light -8 flashes per sec?
steve, October 2015


done
probably diverter valve stuck

stickybun, October 2015
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