I thought I'd make a guide for changing the cabin air filter, since I've just done it in my V40, it didn't hurt to take some photos as I went along.
It's a laughably easy job to change the filter, which can be bought for about £15-20, and saves a little bit of labour cost at service. Took me about 5 minutes to change. As a non-mechanically minded type, I'm always slightly fearful of doing anything to my car without seeing a series of pictures and words first, so here you go. Mine is a UK right hand drive V40 phase 2 (02 plate).
Lo and behold, I've solved my windscreen demist problem! Now lots and lots of air blows up the windscreen, which wasn't happening before, to the extent that I figured the routing flap or linkage must be broken. It clearly isn't, so that was a cheap relief. Judging by the state of the one I took out, I now understand why there wasn't any throughput..... no idea how long it's been there but I suspect it's probably the original filter.
I suspect my passenger foot well cowl has been replaced by the previous owner, it's a different colour (black instead of beige), there are two extra screw holes as if it's come from another car, and I don't know why it's so tatty. But anyway.
EDIT (18/11/2011): the effiency of mine has decreased over the past year. As such it does appear to need changing once a year. I'm yet to remove it to have a look at the condition but there's a lot less air throughput than there was immediately after fitting.
You're looking for two black screws along the top edge to unscrew to release the cowl. There could be a third screw or clip to release (see later) as my cowl seems to be a bit too loose, but this should be easy to find.
Once you've undone the two screws, simply tug the cowl in the direction of the arrow.
It'll come away really easily.
Probably best to put the screws back in the cowl so you don't lose them....!
Dead centre, just below the bottom edge of the glove box, you'll see this label, and a black plastic tab, which is the near end of the cover for the air filter.
Simply push the tab, and pull the front of the cover piece down and then away. The cover rests on a ledge at the back to ensure a good fit.
The cover is foam-backed with a couple of apertures. Give it a quick brush out if it's as filthy as mine!
This is the bottom end of the filter. Note the arrow pointing at the burr, this helped me to work out which way around to put the new filter in.
Simply tug down the black tabs to pull out the old filter.
Note that there are a series of plastic teeth down the left side of the filter. This means the filter will bend safely to the left, which you need to do to remove it, but it will NOT bend the other way!
And this is my old filter removed, first photo is the left (input) side. As you can see it's clogged with black dust, leaf debris and grime from goodness knows how many years of being in there...
...but it's done pretty well at its job, as this photo is the right (output) side which is surprisingly clean considering the other side.
So this is the replacement part, 30612666 which is for V40/S40 phase 2 with air conditioning. This superseded the phase 1 filter part number. There's apparently a different part for non-AC models. This one cost me about £15 delivered from eBay but they don't come up all the time and price from FRF Swansea using Volvo Owners Club internet discount code C3 is £15 + £3 delivery. Make sure yours is a new, boxed Volvo part, if possible, although you can buy aftermarket filters for about £15. I got a sticker with mine so I could record the date and mileage.
Simply shove the new filter back up into the slot, the Volvo logo should be facing you with the teeth up the left side. Make sure you only bend the filter to the left as shown.
This is what happens if you put it in or bend it the wrong way. So don't...
With the new filter fully inserted, simply replace the cover by resting the far end on the ledge, and lifting the near end until the plastic tab clicks into place.
Shove the cowl back into place, making sure you fit the right side above the air vents. Screw the two screws into the metal threads, as shown below.
What I don't know is whether there's another fitting missing. The cowl is very loose, and there's a suspicious looking place where a retaining clip might go, but as you can see, that would leave a suspicious gap around the cowl.