arizona & colorado have been doing diesel emissions inspections for a few
years now.
virginia is adding it now too.
testing is dependant on where you live (county or metro area) more
than anything (at the present time).
testing is performed at different levels, confirmation of OBDII functioning
(no codes stored and the MIL/SES/CEL light is operational...they turn it
on & off to confirm the bulb funtions and you havent disabled the light).
testing also will include visual inspection of exhaust systems too.
additionally opacity tests are also performed via "dyno" roller or LOAD
based testing...which may include a throttle SNAP test and/or some
rpm "hold".
the opacity test is a measurment for BLACK smoke....many states have
been doing road side testing (using lasers) to find/notify AND FINE gross polluter.
ARIZONA ADEQ: Vehicle Emissions TEST INFO: Diesel Vehicles