Missouri requires biennial emissions testing (combined with the state's biennial safety inspection) on vehicles registered in the St. Louis ozone nonattainment area, which includes St. Louis City and Franklin, Jefferson, St. Charles and St. Louis counties.
Diesel Vehicles:
Diesel vehicles must undergo OBD emissions testing in the same way as gasoline powered vehicles. Diesel vehicles with a model year of 1996 or older are exempt.
Exemptions:
1. Vehicles with a model year of 1995 or older
2. New motor vehicles, during the model year of the vehicle and the following year, provided the odometer reading is less than 6,000 miles at the time of the sale
3. Motor vehicles sold for junk, salvage, or rebuilding
4. Vehicles that display historic or street rod plates
7. Vehicles driven fewer than 12,000 miles between biennial safety inspections (must submit a mileage based exemption request)
8. Vehicles with a GVWR of over 8,500 lbs.
9. Vehicles powered exclusively by solar power, electric, hydrogen
For more information visit: https://dor.mo.gov/motorv
Missouri Statutes
§ 307.170. Other equipment of motor vehicles--violations, penalty
2. Muffler cutouts: Muffler cutouts shall not be used and no vehicle shall be driven in such manner or condition that excessive and unnecessary noises shall be made by its machinery, motor, signaling device, or other parts, or by any improperly loaded cargo. The motors of all motor vehicles shall be fitted with properly attached mufflers of such capacity or construction as to quiet the maximum possible exhaust noise as completely as is done in modern gas engine passenger motor vehicles. Any cutout or opening in the exhaust pipe between the motor and the muffler on any motor vehicle shall be completely closed and disconnected from its operating lever, and shall be so arranged that it cannot automatically open, or be opened or operated while such vehicle is in motion.
Custom
Available for vehicles that are at least 25 years old and manufactured after 1948, or manufactured to resemble such vehicles, that have been altered from the manufacturer's original design or have an entire body constructed from non-original materials, and which may be used for occasional transportation, exhibitions, club activities, parades, tours, and similar uses and not for general daily transportation.
Historic
Available for vehicles that are at least 25 years old, owned solely as collector items, and used for exhibition and educational purposes.
Street Rod
Available for vehicles manufactured before 1949, and vehicles manufactured to resemble such vehicles, that have been altered from the manufacturer's original design, or have an entire body constructed from non-original materials, and which may be used for occasional transportation, exhibitions, club activities, parades, tours, and similar uses and not for general daily transportation.
YOM
Vehicles registered as "Historic" may display YOM plates with DMV approval.