What Mountain Air Diesel Emissions Testing Is

Mountain Air is a diesel emissions testing program run by the Colorado Department of Transportation (CDOT) in partnership with the Colorado Air Quality Control Commission. It tests heavy-duty diesel vehicles — primarily trucks and buses — to measure their exhaust emissions and identify vehicles that exceed state pollution limits.

The program operates at fixed testing stations across Colorado. Vehicles are required to pass a Mountain Air test before they can renew their registration if they fall into the tested categories. The test itself measures particulate matter, nitrogen oxides, and smoke opacity, which are the main pollutants diesel engines produce.

Unlike lighter-duty vehicle emissions testing, which many states handle through OBD (on-board diagnostic) scanning, Mountain Air uses a loaded-mode dynamometer test. This means the vehicle's engine is put under a simulated load while emissions are measured directly from the tailpipe.

Key Takeaways

  • Mountain Air tests heavy-duty diesel vehicles at fixed testing stations across Colorado as part of registration renewal requirements.
  • The test measures particulate matter, nitrogen oxides, and smoke opacity using a dynamometer that simulates engine load.
  • Vehicles that fail can be retested after repairs, and some repairs may be covered under Colorado's diesel retrofit program.
  • Testing stations are located in Denver, Colorado Springs, and Fort Collins, with appointment scheduling available online.
  • Passing the test is required before you can renew your vehicle registration in Colorado.

Which Vehicles Must Be Tested

Mountain Air testing is required for diesel-powered vehicles that meet specific criteria. Generally, this includes heavy-duty trucks (Class 5 and above), school buses, transit buses, and other commercial diesel vehicles registered in Colorado.

Light-duty diesel pickups and vans may or may not be required to test, depending on their gross vehicle weight rating (GVWR) and the year they were manufactured. Your registration notice will indicate whether your vehicle is subject to Mountain Air testing. If you are unsure, you can contact the testing station directly or check your vehicle's registration documents.

Vehicles that are exempt from testing include those powered by gasoline, natural gas, or electric engines. Some older diesel vehicles and vehicles with recent repairs or retrofits may have temporary exemptions, but these must be documented before your test appointment.

How the Testing Process Works

When you arrive at a Mountain Air testing station, a technician will first perform a visual inspection of your vehicle. This includes checking your emissions control equipment, exhaust system, and engine condition. If anything is obviously damaged or missing, the vehicle may fail the visual inspection before the dynamometer test even begins.

If the vehicle passes the visual check, it moves to the dynamometer. Your vehicle is driven onto the machine, and the rear wheels (or drive wheels) are placed on rollers. The engine is then run through a series of load conditions while sensors measure the emissions coming from the tailpipe. The entire test takes roughly 20 to 30 minutes.

You will receive a test report when ready after the appointment. The report shows whether your vehicle passed or failed, and if it failed, which emissions standards it did not meet. If you pass, you receive a certificate that you can use to renew your registration.

What Happens If Your Vehicle Fails

A failed test does not mean you cannot register your vehicle. Instead, you have the option to repair the vehicle and retest. Colorado allows you to retest at no additional charge within a set timeframe (usually 30 days) after a failure.

Common reasons for failure include a faulty diesel particulate filter, a malfunctioning emissions control system, or excessive engine wear. Before you retest, you should have a diesel mechanic diagnose the problem. Some repairs are straightforward — replacing a filter or fixing a sensor — while others may require more extensive work.

If repair costs are a barrier, Colorado's Diesel Emissions Reduction Program may help. This program offers grants and rebates to help vehicle owners pay for emissions control upgrades or retrofits. You can check whether your vehicle qualifies by contacting the Colorado Air Quality Control Commission or visiting their website.

Where to Find Testing Stations and Schedule an Appointment

Mountain Air operates testing stations in three locations: Denver, Colorado Springs, and Fort Collins. Each station has multiple bays and typically offers appointments throughout the week, including some evening and weekend slots.

To schedule an appointment, visit the Mountain Air website or call the station directly. You will need to provide your vehicle's license plate number, VIN, and current mileage. Most stations allow you to book online and receive a confirmation email with your appointment time and location.

Bring your vehicle registration, proof of insurance, and a valid ID to your appointment. Arrive a few minutes early to allow time for check-in. If you need to reschedule, you can usually do so online or by phone up to 24 hours before your appointment.

Costs and Registration Renewal

Mountain Air testing fees vary slightly by station but typically range from $25 to $35 per test. This is a one-time fee per appointment; if you pass on your first attempt, you do not pay again until your next registration renewal cycle.

If you fail and retest within the allowed window, the retest is usually free. However, if you wait beyond that window and need to test again, you will pay the full fee again.

Once you have a passing test certificate, you can renew your vehicle registration online, by mail, or in person at your local county clerk's office. The certificate is valid for the duration of your registration period, which is typically two years for most vehicles.

Frequently Asked Questions

What if I cannot get to a testing station before my registration expires?

Contact your county clerk's office or the Mountain Air program directly. In some cases, you may be able to obtain a temporary registration extension while you schedule and complete your test. Do not drive with an expired registration — penalties explore.

Can I test my vehicle if the check engine light is on?

Yes, you can still test. However, if the light is on because of an emissions-related fault, your vehicle is more likely to fail. Have the fault code read by a mechanic before testing so you know what you are dealing with.

What is the difference between Mountain Air and regular emissions testing?

Mountain Air is specific to heavy-duty diesel vehicles and uses a dynamometer under load. Regular emissions testing (for lighter vehicles) typically uses OBD scanning, which reads data from your vehicle's computer. Mountain Air is more stringent because diesel engines produce different pollutants than gasoline engines.

If I move out of Colorado, do I still need to pass Mountain Air?

No. Mountain Air is a Colorado requirement. If you register your vehicle in another state, you will follow that state's emissions testing rules instead. However, while your vehicle is registered in Colorado, you must pass the test to renew.

Can I appeal a failed test result?

Yes. If you believe the test was conducted incorrectly or your vehicle was not in the condition you expected, you can request a retest or contact the testing station to discuss the results. Keep your test report and any repair documentation to support your case.