The service measures state that the air filter should be changed every 30,000 miles.
Personally I think this is silly and I would recommend doing it yearly.
If you don't, at least remove it and give it a vacuum from the side that faces the incoming air.
Undo both of the hose clamps from the air box to the air inlet pipe using a 7mm socket.
Flip down the 3 metal clips across the visible edge of the airbox. Pull the joining hose back and lift the air box lid.
The air box lid can now slide to the right to give you more clearance.
You can now remove the rubber joining hose that connected the inlet pipe to the air box.
The air box lid can now be pulled towards you and angled up to remove it from the engine bay.
Smart have even designed the right side of the air inlet pipe with a flatter section so the lid can pass over it.
Remove the old filter and replace with a new one.
When refitting the air box lid, ensure that the tabs at the back of the box are slid into position
and the lid is sitting flat when you flip the 3 clips back up. Check that the back of the lid is secure.
Non Turbo, Non MHD
Unhook both clips on either side of the airbox, the tops of the clips just pull back.
Then use a screwdriver or a 7mm socket and undo the pipe clamp.
Pull the pipe off of the airbox, you may need a bit of force so swing on it.
There is a final sprung clip underneath the area you just pulled the pipe from.
With that unclipped, lift the edge nearest you about 20mm and pull towards you.
The entire airbox lid will come away from the base.
This shows the standard smart convoluted cotton air filter.
Just above that you can see the cutouts that the airbox lid pushes in to.
Pull the filter out and clean or replace with a new one.
Standard filter available from smart (part number A0010940301).
Much better PiperCross filter available from SmartMods here.
Refitting is the reverse of removal.
Highlighted below are the 2 water drain points, ensure they are not blocked with debris.
Following info by Carl G
Shortly after i bought my fortwo i decided to change the air filter. I discovered that one of
the rubber water drain points (circled in red in the picture above) was missing and that a LOT
of dirt had been sucked in from below. The air filter was full of dirt and needed to be
changed immediately. The car had only 9000 miles on the meter. My recommendation
would be to check that the rubber drain points are in place if you buy a used fortwo.
If a rubber drain point is missing, the smart part number for this item is A 140 832 00 84