Valiant eco tec 637 plus fault f28

[tpat696]

Hi All I have a Valliant 637 plus boiler out of warranty which has a f28 fault code I have had a engineer out who has now changed the gas valve , new circuit board, new cable , new mass flow sensor , new electric rod. Cleaned up internals.Even after all these changes it’s still not working. The suggestion now is a new boiler but i feel there must be something that can possibly be done or maybe something has been missed. The engineer says in his 20 years he has never faced a situation like this. He has also had another plumber come out for a second opinion and nobody can seem to fix it . Any thoughts ?