Enhanced Areas--California's smoggiest urbanized
regions which currently do not meet federal or state air
quality standards for ozone and carbon monoxide. Fifteen
percent (15%) of the vehicles in these areas will have
their biennial Smog Checks at a Test-Only Station. All
vehicles in these areas will be tested for another
unhealthy smog-forming pollutant, NOx, by using a
treadmill-like device called a dynamometer.
http://www.smogcheck.ca.gov/StdPage.asp?Body=/geninfo/factsheets/Enhanced_Smog_Check_Program_Overview-June_1998.htm
Cheers,
Joe Garcia