Morning,

Over the last few days, I've been getting an intermittent fault in my
boiler (a Chaffoteaux Brittony combi). The code it blinks translates to
"fan on but air pressure switch not activated", or words to that effect.
Turning the power off and on again (can you tell I'm a software
engineer? :-) ) clears it for some hours / up to a day.

I have the service manual for it, and can see that the air pressure
switch and venturi (and the fan) is mounted inside the "sealed chamber"
(but not in the combustion chamber itself). I'm satisfied with my
competence to open this up (and in any case it's not actually
gas-carrying) but I'm not sure what I'd do when I get there if nothing's
obviously damaged.

It seems to me that the switch is not necessarily the culprit - it could
be doing its job perfectly and warning that the pressure is not changing
due to some other fault. So I don't want to just order another switch
and slap it in.

Any suggestions on how to proceed?

Cheers,

Pete