What Colorado's emission test measures and who has to take it

Colorado requires an emission test for most vehicles registered in the state, and the test measures how much pollution your car's exhaust produces. The state runs this program through Emissions Testing Colorado, which operates testing stations across the Denver metro area, Colorado Springs, and Fort Collins. If you live in one of these regions, you must pass an emission test before you can renew your vehicle registration.

The test itself is straightforward: a technician plugs a diagnostic scanner into your vehicle's onboard computer port (located under the dashboard on the driver's side) and reads emissions data. The scanner checks whether your engine is burning fuel cleanly and whether your catalytic converter and oxygen sensors are working. The entire test takes about 10 minutes and costs between $20 and $30, depending on the testing station.

Not all Colorado vehicles need testing. Vehicles registered outside the Denver metro area, Colorado Springs, and Fort Collins are exempt. Diesel trucks, motorcycles, and vehicles older than 1975 are also exempt. If your vehicle is newer than the current model year minus 3 years, you may may have access to for a waiver in some cases—check with your local testing station about age-based exemptions.

Key Takeaways

  • Colorado's emission test plugs into your vehicle's onboard computer and takes about 10 minutes, costing $20 to $30 per test.
  • You must pass the test before renewing your registration if you live in the Denver metro area, Colorado Springs, or Fort Collins.
  • A failed test usually means your catalytic converter, oxygen sensors, or engine computer needs attention—not a complete engine overhaul.
  • You can retest at any station after repairs, and most stations will hold your test results for 60 days so you have time to fix problems.
  • Vehicles older than 1975, motorcycles, and diesel trucks are exempt from testing in Colorado.

Why your vehicle fails the emission test

A failed emission test almost never means your engine is broken. It means one or more emissions control systems is not working as designed. The most common reason for failure is a faulty oxygen sensor, which tells your engine how much fuel to burn. When an oxygen sensor fails, your engine runs too rich (too much fuel) or too lean (too little fuel), and the exhaust becomes dirtier. Replacing an oxygen sensor typically costs $150 to $300 and is one of the quickest fixes.

The second most common cause is a problem with your catalytic converter, the part that chemically breaks down harmful gases in your exhaust. A failing catalytic converter will trigger a check engine light and cause your vehicle to fail the emissions test. Catalytic converter replacement is more expensive—usually $400 to $1,200 depending on your vehicle—but it is a real safety issue because a failing converter reduces your engine's efficiency and can damage other parts.

A third common reason is a loose or missing gas cap. This sounds minor, but a loose cap allows fuel vapors to escape, which the onboard computer detects as a leak in the fuel system. Tightening or replacing the cap costs almost nothing and sometimes fixes a failed test on the spot. Some testing stations will retest you when ready if you fix the cap while you are there.

Less commonly, a failed test points to a problem with your engine computer, a faulty mass airflow sensor, or a malfunctioning evaporative emissions system. These are more complex repairs, but they are still specific problems with known solutions—not signs that your vehicle is unsafe to drive.

What happens after you fail and how to retest

When you fail an emission test, the testing station gives you a printed report that shows which emissions systems failed and why. You do not have to fix the problem when ready. Most testing stations hold your test results for 60 days, which gives you time to take your vehicle to a mechanic, have the repair done, and return for a retest without losing your test record.

After you have the repair completed, you can return to any Emissions Testing Colorado station to retest. You do not have to go back to the same station. Bring your vehicle and your failed test report; the technician will run the diagnostic again and, if your vehicle now passes, you will receive a passing certificate. The retest costs the same as the initial test—$20 to $30.

If you pass the retest, you can then renew your vehicle registration with the Colorado Division of Motor Vehicles. You will need your passing emissions certificate and your current registration documents. You can renew online through the DMV website, by mail, or in person at a local DMV office. Registration renewal costs vary by vehicle type and county but typically range from $100 to $200 for a standard passenger vehicle.

If you fail a second time, the testing station will give you another report. At this point, you should take your vehicle to a trusted mechanic and bring the failed test report with you. The report tells the mechanic exactly which systems are not working, which makes the diagnosis faster and more accurate than a general check-engine light.

Finding a testing station and scheduling your test

Emissions Testing Colorado operates stations in three regions: the Denver metro area (including Aurora, Boulder, and surrounding counties), Colorado Springs, and Fort Collins. You can find the complete list of testing stations and their hours on the Emissions Testing Colorado website. Most stations are open Monday through Friday during business hours, with some offering Saturday appointments.

You do not need an appointment at most stations—you can straightforward drive in during operating hours and wait your turn. Wait times vary by location and time of day; stations in the Denver metro area tend to be busier during lunch hours and late afternoon. If you want to avoid a wait, call the station ahead of time to ask about current traffic.

Bring your vehicle registration and a valid photo ID. The technician will verify that the vehicle you are testing matches your registration. If you are retesting after a repair, bring your failed test report as well—this helps the technician understand what was previously wrong and confirms that you are the same vehicle.

How to prepare your vehicle for the emission test

Your vehicle should be in normal operating condition before you take it for an emission test. This means the engine should start easily, run smoothly at idle, and not have any obvious problems like rough idling, stalling, or a check engine light that has been on for weeks. If your check engine light is on, the test station will still test your vehicle, but you will likely fail because the light indicates a problem with an emissions system.

Before your test, check that your gas cap is tight and not cracked. A loose or damaged cap is one of the easiest things to fix and one of the most common reasons for failure. If your cap is damaged, replace it—a new cap costs $5 to $15 at any auto parts store.

Make sure your vehicle has been driven recently. Emission tests work best when your engine is warm and has completed a full drive cycle (typically 10 to 15 minutes of mixed city and highway driving). If your vehicle has been sitting for days, drive it for at least 20 minutes before your test appointment. This allows the onboard computer to run its self-diagnostic and clear any temporary fault codes that might cause a false failure.

You do not need to have your vehicle serviced or detailed before the test. The emission test does not check your vehicle's appearance, maintenance history, or mechanical condition beyond what the onboard computer reports. A dirty or old vehicle will pass if its emissions systems are working correctly.

What to do if you cannot pass the test

If you have had your vehicle repaired and it still fails the emission test, you have a few options. First, take your vehicle back to the mechanic who did the repair and show them the failed test report. The mechanic may have missed something or the repair may not have been done correctly. Most reputable shops will redo the work at no charge if the repair did not fix the problem.

Second, you can take your vehicle to a different mechanic for a second opinion. Bring the failed test report with you so the new mechanic knows exactly which systems are not working. A second opinion costs money, but it can save you from paying for unnecessary repairs.

Third, if your vehicle is very old or the repairs are very expensive, you may want to explore whether your vehicle qualifies for a vehicle buyback program or trade-in information. Some Colorado counties offer programs that help owners of older, high-emission vehicles retire their cars and purchase cleaner vehicles. These programs vary by county and are not available everywhere, so contact your local air quality office to ask whether your county has one.

Finally, if you believe the test result is wrong, you can request a retest at a different testing station. Testing equipment can occasionally malfunction, and a second test at a different location will confirm whether the failure is real or a testing error. This costs another $20 to $30, but it gives you a definitive answer.

Emission test requirements for vehicle registration renewal

In Colorado, you cannot renew your vehicle registration without a passing emission test certificate if your vehicle is registered in the Denver metro area, Colorado Springs, or Fort Collins. The Colorado Division of Motor Vehicles will not process your renewal until you provide proof of a passing test.

Your emission test certificate is valid for the calendar year in which you take the test. If you test in January, your certificate is good through December 31 of that year. If you test in November, your certificate is still good through December 31, so you do not need to retest until the following year. Plan your test timing so that you have your certificate before your registration expires.

When you renew your registration, you will need your passing emissions certificate, your current registration documents, and proof of insurance. You can renew online through the Colorado DMV website, by mail, or in person at a local DMV office. Online renewal is usually the fastest option and takes a few minutes.

Frequently Asked Questions

Can I drive my car if it fails the emission test?

Yes. A failed emission test does not make your vehicle unsafe to drive or illegal to operate. It only prevents you from renewing your registration. You can drive your vehicle to a mechanic, to a testing station for a retest, or anywhere else while you are working on the repair. You just cannot legally register it until it passes.

What if I move out of Colorado or out of the testing area?

If you move to a county that does not require emission testing, you do not need to pass a test to renew your registration there. However, if you move within Colorado to another county that does require testing, you will need to pass the test before you can register your vehicle in that county. Contact your new county's DMV office to confirm whether testing is required.

How often do I have to take the emission test?

You must take the emission test once per year when you renew your vehicle registration. If your vehicle passes, you do not need to test again until the following year. If it fails, you can retest as many times as needed within the 60-day window that your test results are valid.

Do I have to use Emissions Testing Colorado, or can I go to a private mechanic?

You must use an official Emissions Testing Colorado station. Private mechanics cannot perform the official state emission test, though they can diagnose and repair emissions problems. The state test is the only one that counts toward your registration renewal.

What if my vehicle is too new or too old to test?

Vehicles newer than the current model year minus 3 years may may have access to for a waiver depending on your county. Vehicles older than 1975 are exempt from testing. Contact your local testing station or your county's air quality office to confirm whether your specific vehicle is exempt based on its age and model year.