As far as emissions testing in the state of Colorado is concerned, you may be in luck and not be required to undergo testing, or you may need to get tested. Weld, Larimer and El Paso counties DO NOT require emissions tests for gas (petrol) vehicles. They DO however, require testing for DIESEL vehicles. In Denver and the surrounding areas they still require emissions testing for all vehicles.
Diesel: Currently, heavy duty and light duty diesel vehicles (this includes everything from a diesel Jetta, to an Eighteen Wheeler Cab) are exempt for the first four model years, however after this they are required to be tested. Light duty vehicles that are a 2003 model or earlier must have a test every year, and light duty vehicles that are a 2004 model year or later must have a test every two years. A heavy-duty vehicle that is over ten model years must have a yearly test, and heavy-duty vehicles fewer than ten model years must have a test every other year. Testing facilities may charge up to one hour of labor rates for test, as there are no set rates for diesel vehicles. Testing facilities can be found in the phone book under automotive, or on the Department of Revenue page of the Colorado.gov web page (link found at the end of the article).
Gasoline: As previously noted, if you drive a gasoline-powered vehicle, you may be exempt from testing. Weld, Larimer and El Paso counties do not require testing, however Denver Metro and surrounding areas DO require testing. For those living in Adams county or Arapahoe county, you may or may not be exempt and the best course of action is to call 303-654-6010 (Adams) or 303-795-4500 (Arapahoe) for information. Test for vehicles that are 1981 or older are $15, and for vehicles that are 1982 or newer are $25. Again, testing facilities can be found in the phone book under automotive, or on the Department of Revenue page of the Colorado.gov web page.
Waivers and Failures: If a vehicle fails an emissions test, the owner is required by law to attempt to repair the vehicle to pass the test. If the vehicle was made in 1968 or later and the owner has spent $715 or more in emissions repairs, the vehicle qualifies for a waiver. If the vehicle was made in 1967 or earlier and the owner has spent $75 or more on emissions relate repairs, the vehicle qualifies for a waiver. The waivers allow the owner to register the vehicle and drive it for one emissions cycle, thus there is time to make the required repairs. Knowingly operating a vehicle with a missing or tampered with emissions control device (a catalytic converter) is a violation of state and federal laws and therefore not eligible for a waiver. Vehicles that fail the test due to smoke are also not eligible for a waiver.
For more information see http://www.colorado.gov/cs/Satellite/Revenue-MV/RMV/1185957917898