How hard are these damn things to replace ? - I had it replaced under warranty at about 3 years old from new, and its gone again.
There is definitely no shortage of oxygen, but I believe they can be blocked by dust or airborne crud - something our house seems to suffer an excess of due to two large muddy dogs :-} (re: shortage of oxygen - the damn doors are never shut as they have free run of the house and garden, which is why we were so sodding desperate to get central heating installed...)
Is it possible to remove and clean up the component in question ?