The full counties of Boulder, Broomfield, Denver, Douglas and Jefferson and portions of Adams, Arapahoe, Larimer, and Weld counties require a passing emission test to register a vehicle.
Diesel Vehicles:
All diesel vehicles model years 2007 – 2013 need an emission test biennially and all diesels model years 1986 – 2006 need emissions tests annually.
Exemptions:
1. Vehicles that are seven (7) model years old and newer
2. Vehicles in their "off" year cycle - vehicles require an inspection every other year.
3. All-electric vehicles (hybrid-electric vehicles are inspected after 7 years).
4. Collector plated vehicles (model year 1975 and older).
5. Horseless carriages, street rods, and kit cars
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42-4-225. Mufflers - prevention of noise
(1) Every motor vehicle subject to registration and operated on a highway shall at all times be equipped with an adequate muffler in constant operation and properly maintained to prevent any excessive or unusual noise, and no such muffler or exhaust system shall be equipped with a cut-off, bypass, or similar device. No person shall modify the exhaust system of a motor vehicle in a manner which will amplify or increase the noise emitted by the motor of such vehicle above that emitted by the muffler originally installed on the vehicle, and such original muffler shall comply with all of the requirements of this section.
(1.5) Any commercial vehicle, as defined in section 42-4-235 (1) (a), subject to registration and operated on a highway, that is equipped with an engine compression brake device is required to have a muffler.
(2) A muffler is a device consisting of a series of chamber or baffle plates or other mechanical design for the purpose of receiving exhaust gas from an internal combustion engine and effective in reducing noise.
(3) Any person who violates subsection (1) of this section commits a class B traffic infraction. Any person who violates subsection (1.5) of this section shall, upon conviction, be punished by a fine of five hundred dollars. Fifty percent of any fine for a violation of subsection (1.5) of this section occurring within the corporate limits of a city or town, or within the unincorporated area of a county, shall be transmitted to the treasurer or chief financial officer of said city, town, or county, and the remaining fifty percent shall be transmitted to the state treasurer, credited to the highway users tax fund, and allocated and expended as specified in section 43-4-205 (5.5) (a), C.R.S.
(4) This section shall not apply to electric motor vehicles.
5 Year Collector
Available for passenger cars and trucks that do not exceed sixteen thousand pounds empty weight, and must be (a) Model year 1975 or earlier; or (b) Model year 1976 or later that was registered as a collector's item prior to September 1, 2009; except that a vehicle so registered is not eligible for registration as a collector's item upon sale or transfer to a new owner; OR (c) A model year at least thirty-two years old unless the vehicle was registered before September 1,2009, and meets the requirements of paragraph (b) of this subsection. If the vehicle is being registered under this paragraph (c) and in the program area, as defined in section 42-4-304: (i) The vehicle must have passed an emissions test meeting the standards of part 3 of article 4 of this title within the last twelve months before being initially registered by the owner as a collector's item; and (ii) The owner must sign an affidavit DR 2839 that the vehicle will not be driven on roadways for more than four thousand five hundred miles per year. All taxes and fees shall be five times the annual taxes and fees normally assessed.
Horseless Carriage
Available for passenger cars and light trucks that do not exceed sixteen thousand pounds empty weight that are at least 50 years old that are highly valued principally because of their early date of manufacture, design, historical interest, or value as collector's items. Vehicles are limited to driving to and from assemblies, conventions for demonstrations and parades, occasional leisure drives, and on other occasions when their operations will not constitute a traffic hazard. Plates are registered for a set 5-year cycle with all plates expiring on the same date within that set 5-year cycle regardless of the date of issue.
Street Rod
Available for vehicles manufactured in 1948 or earlier with body designs that have been modified for safe road use.
YOM
Vehicles that are least 30 years old may display YOM plates with DMV approval.