Viessmann vitodens 100 F4 error won't go away...

I'm kind of getting to the end of my tether on this one as I would appreciate so experienced advice please. Our viessmann 100 boiler started f4 erroring about 2 years ago. Press reset and it'll work for an hour, maybe 2 then errors again.
The burner unit looks like new, it's very clean. So far we've had the igniter unit, electrode, PCB and gas valve replaced but still it keeps erroring. The gas pressure is by the book as is the electrode gap and there is no condensate problem.
I've spoken to viessmann direct and they suggested changing everything we have had changed so their view now is 'tough luck, it's out of warranty'. Having spent half the cost on these repairs of a new boiler, do I have to bite the bullet and actually get a new boiler now?
Grateful for any suggestions...thanks in advance.

Hotblack Desiato said:

and there is no condensate problem.

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I would double check this, they are terrible for blocking in the sump under the burner, also when the condensate backs up it will erode the insulation panels, unfortunately to check the condensate properly you would have to be GSR and it is against site rules to advise you how to do it

I would get someone in to check or remove the jimmy flapper.

They were prone to stick shut on older models

Thanks both for your advice, I don't do any work myself, I have a local long suffering plumber to do that. I presume he'll know what the Jimmy flapper is will he? Cheers

That’s all I know it as. It sits just after the fan

Rubber non-return flap on the outlet from the fan. IIRC it was fitted on early models of the 100 but later discontinued as it tends to stick closed.