Never heard of this until now...
Had a bit of a google...
http://www.smogtips.com/remote_sensing.cfm
In this link it advises that people would be mailed literature and invites to have a 'vehicle repair' which sounds like a rectificatio scheme rather than penalty scheme (all be it I imagine with slightly larger costs than a ticket would be)
Presume that stop checkpoints would be the same approach as these mobile ones?
Also what are the limits etc. Surely a modern car, even with modifications, should be well below limits?