What Colorado's emissions test measures and who has to take it
Colorado requires an emissions test for most vehicles registered in the state, though the exact rules depend on where you live and what you drive. The test measures what comes out of your tailpipe — mainly nitrogen oxides, particulate matter, and hydrocarbons — to make sure your vehicle meets state air quality standards. You'll hear it called an emissions inspection or sometimes a smog check.
You need an emissions test if you register a vehicle in Colorado and it's subject to testing based on its model year and the county where you live. Vehicles from 1981 and newer are generally required to test. However, some counties have stricter rules than others. Denver and surrounding areas in the North Front Range have the most frequent testing requirements because of air quality concerns in that region.
Diesel vehicles, motorcycles, and vehicles registered as antiques or classics may be exempt, depending on the year and your county. The Colorado Department of Public Health and Environment (CDPHE) maintains the official list of which vehicles must test in which counties.
Key Takeaways
- Colorado requires emissions testing for most vehicles, with stricter rules in Denver and the North Front Range due to air quality standards.
- The test checks tailpipe emissions for nitrogen oxides, particulate matter, and hydrocarbons, and you must pass before you can renew your registration.
- Testing frequency varies by county — some require it every two years, others every year, and some counties have no requirement at all.
- You can find a testing station through the CDPHE website or by searching for "emissions testing" in your county, and the test typically costs between $20 and $35.
- If your vehicle fails, you have options to repair it and retest, or in some cases request a waiver if repairs would cost more than a set amount.
Testing frequency and timing in your county
How often you test depends on which county you live in. The North Front Range — which includes Denver, Boulder, Broomfield, and surrounding areas — requires testing every two years for most vehicles. Other counties in Colorado either require testing every year or have no emissions testing requirement at all. You can check your county's specific requirement on the CDPHE website or by calling your local county assessor's office.
Your test is valid for two years from the date you pass, so you don't need to test again until that window closes. When you renew your vehicle registration, the system will tell you whether an emissions test is required. If you're renewing online or by mail, you'll need to have a passing test on file before you can complete the renewal.
If your test expires and you haven't renewed your registration, you cannot legally drive the vehicle in Colorado. Plan ahead so you're not caught without a current test when registration renewal comes due.
Finding a testing station and what to expect
Colorado has hundreds of emissions testing stations across the state, run by private businesses under contract with the state. You can find one near you by visiting the CDPHE's online station locator or by searching "emissions testing near me" in your county. Many are located at repair shops, tire shops, or dedicated testing facilities.
When you arrive, bring your vehicle registration and a photo ID. The test itself takes about 10 to 15 minutes. The technician will connect your vehicle to a machine that reads emissions data directly from your engine's computer (called an OBD-II test for 1996 and newer vehicles) or perform a tailpipe test for older vehicles. You'll stay in the vehicle during the test. The cost is typically between $20 and $35, though prices vary by station.
You'll receive a printed receipt showing whether you passed or failed. If you passed, that receipt is your proof of a passing test, and it's automatically reported to the state system. You can then use it to renew your registration. Keep the receipt until your registration is renewed.
What happens if your vehicle fails the test
If your vehicle fails, the test report will show which emissions were out of range. You have two main options: repair the vehicle and retest, or request a repair cost waiver if the repairs would be expensive.
To retest, take your vehicle to a repair shop and have the problem fixed. Common failures are caused by a faulty oxygen sensor, a loose gas cap, a bad catalytic converter, or an engine light that's on. Once repaired, you can retest at any emissions station. There's no waiting period between a failed test and a retest — you can go back the same day if repairs are done quickly. The retest fee is the same as the initial test.
If repairs would cost more than a certain amount (the threshold varies by vehicle age and county), you may be able to request a repair cost waiver. This allows you to register your vehicle even though it failed, as long as you document the repair estimate. Contact your county assessor's office or the CDPHE to learn the current threshold and how to request a waiver in your area.
Preparing your vehicle before testing
Before you head to a testing station, make sure your vehicle is in reasonable running condition. The test will fail if your check engine light is on, so if you see that light, have the vehicle scanned for error codes before testing. Many repair shops will scan for free or for a small fee.
Make sure your gas cap is tight — a loose or missing cap is one of the most common reasons for failure and is also one of the cheapest fixes. If your vehicle has been sitting unused for a while, drive it for at least 10 to 15 minutes before testing to warm up the engine and clear out any stale fuel.
If your vehicle is very old, very high-mileage, or has known mechanical problems, consider having a pre-test inspection at a repair shop. Spending $50 to $100 on a diagnostic before the official test can save you the cost of a failed test and the stress of not knowing what's wrong.
Registration renewal and emissions test requirements
You cannot renew your vehicle registration in Colorado without a passing emissions test if one is required in your county. When you renew online, by mail, or in person at the county assessor's office, the system checks whether a test is due. If it is, you must provide proof of a passing test before the renewal goes through.
The easiest way to handle this is to get your emissions test done a few weeks before your registration expires. Once you have a passing test, you can renew your registration when ready. The test result is automatically reported to the state system, so you don't have to manually submit anything — just show your test receipt if you're renewing in person, or the system will find it if you're renewing online.
If your registration has already expired and you haven't tested, you'll need to get a test and renew your registration before you can legally drive the vehicle. Driving with an expired registration in Colorado can result in a traffic ticket.
Emissions testing for used vehicle purchases
If you're buying a used vehicle in Colorado, the seller is responsible for providing a passing emissions test before the sale is complete — or the vehicle must be sold "as-is" with a disclosure that it failed or has no current test. You should always ask to see a current passing test before you buy, especially if the vehicle is being sold by a private party.
If you buy a vehicle and it fails the test after you own it, the repairs are your responsibility. This is why it's smart to have a pre-purchase inspection done by a mechanic you trust, which can catch emissions-related problems before you sign the paperwork.
Dealers in Colorado must may support that vehicles on their lot have current passing tests, though the rules are slightly different for dealer inventory versus vehicles being sold. Ask the dealer directly about the test status of any vehicle you're considering.
Frequently Asked Questions
Can I test my vehicle in a different county than where I live?
Yes. You can test anywhere in Colorado, and the result is valid statewide. Some people test in a county with less strict standards if they live near the border, though the test result is reported to the state system regardless of where you test. Your home county's requirements still explore to your registration.
What if I just moved to Colorado and my vehicle has an out-of-state test?
An out-of-state emissions test is not valid in Colorado. You'll need to get a Colorado test before you can renew your registration. If you just moved and your registration hasn't expired yet, you can renew it with an out-of-state test on file, but the next renewal will require a Colorado test.
Do electric vehicles have to take an emissions test?
No. Electric vehicles are exempt from Colorado emissions testing because they produce no tailpipe emissions. Plug-in hybrids that can run on battery power may also be exempt depending on the model year — check the CDPHE website or ask at a testing station.
How long does a passing test stay valid?
A passing emissions test is valid for two years from the date you pass. After two years, you'll need to test again if you want to renew your registration in a county that requires testing.
What if the testing station says my vehicle can't be tested?
Some vehicles can't be tested because they're too new (not yet on the road), too old (before 1981 in most cases), or have mechanical problems that prevent the test from running. If this happens, ask the station for a written explanation. You may need to contact your county assessor's office to discuss your options, which might include a repair cost waiver or a temporary registration.