Vokera Excell 80E. Hot water problem

I have a Vokera Excell 80E.Can get no hot water when the selector in on the water setting.The red flue diagnostic light keeps flashing.Have checked the flue is clear and the fan is working.Can get hot water if I switch the selector switch to central heating and switch on the heating.
PCB(circuit board) and rubber diaphrams replaced last year.Can anyone help please.

It could be the PCB .

There,s a thingamajig you can get to test the PCB. I think its called a multimeter which has various settings so you can see whats doing what and their not that expensive either.

Flow switch diaphragm, always more resistance on hot water side of primary especially on older boiler, so a stretched diaphragm first shows up on hot water demand. This is why you get hot water when you have heating on, as there is less resistance on the heating circuit of the primary so there is sufficient flow to move a slack diaphragm, once boiler lit in heating mode if you run a tap it just keeps lit, but diverts flow to domestic heat ex.

First attempt to fix is change diaphragm but may also be sludged domestic heat ex primary side, main heat ex or tired old pump.

Not a problem now as you need heating on anyhow. My prescription, change diaphragm and treat system water with ferroquest (Sentinel X800) for full 3 hours drain out refill run 1 hr, drain out refill with Inhibitor of your choice.

Thanks to all who replied.Problem now resolved.Faulty microswitch.