All vehicles registered in Davis, Salt Lake, Utah and Weber counties with model years less than six years old are required to have an emission test once every two years. Vehicles in this category that have even-numbered model years must have an emission test in even-numbered years, and vehicles that have odd-numbered model years must have an emission test in odd-numbered years. Vehicles with model years six years old and older (to 1968) must have an emission test every year.
Cache County vehicles with model years six years old and greater that have even-numbered model years, must have an emission test in even number years, and vehicles with odd-numbered model years must have an emission test in odd-numbered years. Vehicles with model years 1968 or older do not require an emission test.
Diesel Vehicles:
Diesel vehicles are treated the same as gasoline powered vehicles for emission inspection purposes except emissions tests are not required for diesel vehicles registered in Utah county.
Exemptions:
1. New Vehicles
2. Pre-1968 model year vehicles
3. Vehicles registered as "vintage vehicles" or "custom"
For more information visit: https://dmv.utah.gov/
Utah Code
§ 41-6a-1626. Mufflers -- Prevention of noise, smoke, and fumes -- Air pollution control devices
(1) (a) A vehicle shall be equipped, maintained, and operated to prevent excessive or unusual noise.
(b) A motor vehicle shall be equipped with a muffler or other effective noise suppressing system in good working order and in constant operation.
(c) A person may not use a muffler cut-out, bypass, or similar device on a vehicle.
Vintage
Available for vehicles that are at least 30 years old and primarily collector's items. Vintage vehicles may be used for participation in club activities, exhibitions, tours, parades, occasional transportation and similar uses but not for general daily transportation.
Custom
Available for vehicles that are at least 25 years old and primarily collector's items. Vehicles registered as custom have a model year after 1948 or were manufactured to resemble a vehicle that is at least 25 years old and has been altered from the manufacturer's original design or has a body constructed of non-original materials. Custom vehicles may be used for participation in club activities, exhibitions, tours, parades, occasional transportation and similar uses but not for general daily transportation.
YOM
Vehicles manufactured in 1973 or earlier may display YOM plates with DMV approval.