17 of 254 counties in Texas require emissions testing, with these all being in the large metropolitan areas:
Houston-Galveston-Brazoria Program Area: Brazoria, Fort Bend, Galveston, Harris, and Montgomery Counties
Dallas-Fort Worth Program Area: Collin, Dallas, Denton, Ellis, Johnson, Kaufman, Parker, Rockwall, and Tarrant Counties
Austin Area: Travis and Williamson Counties
El Paso Area: El Paso County
Motorists must successfully pass both the emissions and safety portions of the inspection to obtain an inspection sticker'
Gasoline vehicles model-year 2 through 24 years old are inspected annually beginning with the vehicle's second anniversary
The above extracted from TCEQ (Texas Commission on Environmental Quality) website.