Here ya go... I pulled this info from the TX DPS website:
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Which vehicles are required to have the emissions test?
1. Vehicles registered in designated county. Designated counties include:
Brazoria, Fort Bend, Galveston, Harris, Montgomery, Collin, Dallas, Denton, Ellis, Johnson, Kaufman, Parker, Rockwall, Tarrant, Travis, Williamson and El Paso counties.
2. Gasoline powered vehicles. Diesel powered vehicles and motorcycles are exempt from the emissions standards, but are still required to have the annual safety inspection.
3. Vehicles between 2 - 24 years old OR upon expiration of the 2 year initial inspection sticker