I've had this problem too.
Whatever I do I can't get any air from the face vents, but the next morning it's fine.

 

Well I'm glad it's not just mine!

I've checked and cleaned the humidity sensor but can't find any other sensors in the cabin that might be faulty.

 

Is that the one just above the glove box? ?little fan type

Sent from my SM-A310F using Tapatalk

 

Yes that's the one. On the 407 Coupe it's attached to the stainless trim just behind the little slots. The trim prises off quite easily without the usual self-destruction of PSA plastic clips so goes back on too!

Mine had 9 years of dust but the fan came on quickly once the ignition was turned on.

 

After another uncomfortable 250 trip yesterday with very little air from the vents I checked the pollen filter again this morning and found it damp.

Not sure how close the filter is to a matrix but it anyway seems more damp on the upstream side.

So now I'm trying to remove the wipers and release the scuttle panel to see if by any chance the fresh air inlet is full of wet leaves and possibly has a blocked drain.

I think it quite possible that there is a sensor that closes the flaps down when there's moisture in the ducts. The only reference I've found to this is in regard to the Citroen C6 which has an 'air quality' sensor. Being built on the same floorpan as the C6 the Coupe may just share some more of the aircon than the other 407 models.

 

Just checked mine and it's bone dry

Ha, yes I was reading Everything Citroen C6 - C6owners too since there a lot of parts in common -- but the snooty C6 owners get very upset if you point this out.

 

Well I managed to lift enough of the scuttle panel to vacuum out quite a lot of leaves but they were very dry.

The pollen filter sits right in front of the matrix. I ran the aircon for several minutes without a filter and of course condensate forms on the matrix.

So I'm left wondering whether the filter got wet because I ran the aircon for 1.5 hours last night with the flaps closed and nowhere for the moisture to go or if the matrix is leaking. I hope it's not the latter as that appears to mean having the dashboard out again.

I'd like to find the drain for the heater box. Presumably there's a rubber pipe somewhere that might be blocked.