In message , Pete Verdon
d writes
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?

You could always check the APS

Check the closed and open contact resistance - you want it to be in the
region of an ohm or less, this involves checking your probe resistance
first. You can activate the APS by GENTLY sucking or blowing on one of
the rubber tubes to the APS. Kink the tube and leave it kinked for 30
seconds or so to check that the APS membrane is not ruptured.Also check
for a split or suchlike in the rubber tubes

check the seals, check the flue,

Are you experiencing the high winds we are getting around here ? If your
flue is exposed, you might be getting blowback

--
geoff