finally check the securing bracket holding fan to condense heat exchanger(this often slips loose causing fan to suck gas out of appliance to fast causing intermittant 01). .
you will have trouble finding that on a linea 28.
Apologies, I was picturing a syntesi, the rest is accurate.
Also sorry 4 postin gas advice, my 1st post on the forum, I should have read the rules, although this info is in instructions left with user & if I'd realised it was 2 1/2 yr ago I wouldn't have bothered