Just behind the battery is a black cover held on by 3 screws.
1. Unscrew these and lift the cover off.
2. Take the cover off.
3. You will see 3 'grey half moon' levers in a row.
4. Spray these with WD40 or equivalent and gently wipe them over with a dry cloth.
5. Move these levers a couple of times [gently] with your fingers.
6. Turn the engine on and leave running for about 5 mins.
7. Replace cover and screw down.
This is what I did when I had the Pollution Fault message come up on my 407, and it worked. Message gone and has stayed gone.