Pete has written it in very good short and simple way...mine is in detail
I managed to fix my Zafira's air vent problem ( I have sold my car but putting comments on to help other people).
Here is what I did:-
How to diagnose the problem. Please switch off your car and take the keys out so nothing is switch on.
Turn the vent setting knob carefully and try to hear plastic to plastic or plastic to metal clicking noise or ticking noise. At some point you will hear it when you turn the air vent setting knob.
If you can hear it then detect where it is coming from. In my case it was in middle of my dashboard where air vent opening mechanism is.
For this you will have to follow the black cable which goes from the vent setting knob to the mechanism. This cable is like accelerator cable black on top and silver in middle which open- close air vent paths.
You will have to remove driver side foot well air pipe connections. Once you have removed the cover and air pipe which has few hole to give the air, you should be able to see the air vent mechanism with torch. if you have not been able to see you might have to remove some more things but I didn't have to. I only remove couple of plastic part that were in the way.
Now if air vent mechanism is visible to you this means 50 percent of your job is done.
Now while keeping an eye on mechanism turn the setting knob. You will see 5-6 cm long and about one to one & half CM wide (similar size to one of your finger) moves. One side of this plastic part should stay in one place and when you move setting knob only one side should move with Cable movement to open air paths.
In my case one side that was not supposed to move was moving- broken which didn't let the other side of this part move enough to be able to open air vent towards screen. I think this is the problem with most Zafiras.
(Example) In my previous post I gave the example like wrist watch. So like on your wrist watch all the handle move and go round but middle mechanism which has all handles attached keeps them in middle.....Imagine if there was a string on the other side of watch handle which makes handle move but in the middle there is only a nail that keep them in middle...and what if this nail breaks and string is still trying to pull the handle what would not work correctly. (I really hope this makes sense if not see below)
One side that should stay in its place is held by a plastic pole -nail. This pole-nail is about 2-5 mm thick and breaks or comes off from its place due to strain of user turning knob the opposite way than it is described in owner's manual. It is a quite common mistake by most drivers that if we want to go to next setting on either side we will move the setting knob quickly there when it might be wrong move I.e. opposite side rather that going round as prescribed in manual. For example if your watch can only go forward but you need to set the time just 1 hour back. you have no choice but to go round and reset your time but if you force your watch to go back 1 hour what happens...............(if you don't know, it breaks)
Fixing and Preventing.
Once you have found the problem it is easy to cure. You can either use silicon based glue or super glue to fix the broken plastic pole- nail. BEST THING IS TO GO TO MOTOR SCRAP YARD AND FIND THIS PART FROM A ZAFIRA (if that Zafira does not have the same issue) or you might be able to find same part on other Vauxhall cars but make sure it matches the part you have taken off.
In my case I had to temporally fix it with glue and when I found good part from scrap yard from Zafira. (I didn't look in other models but I guess it may be same on other Vauxhall models.
Once you have put new -fixed part in PLEASE ENSURE THAT YOU ONLY TURN THE KNOB ONE WAY AS IT IS DESCRIBED IN OWNERS MANUAL.
I have not gone into details how to take it off and put it on. If you have time in your hand you can go to scrap yard and have a go there before doing your own car. It is quite easy to do if you are good at fixing things. It is not easy to describe as to how to take is off and put it on and it has been nearly 3 years since I sold mine.
While you are at it clean or change pollen filter.
Good luck.
Mamoon
March 2016