The 16-minute smog test checks whether your car's emissions stay within your state's limits during a short, controlled drive cycle

The 16-minute smog test, also called the IM240 test or dynamometer test, runs your car on a treadmill-like machine called a dynamometer while a computer measures what comes out of your tailpipe. The test lasts about 16 minutes and simulates real driving — the machine speeds up and slows down to mimic city and highway conditions. Your car passes if its emissions of nitrogen oxides, particulate matter, and other pollutants fall below the thresholds your state has set.

Not every state uses the 16-minute test. Some states use the OBD test (on-board diagnostics), which reads data from your car's computer instead of measuring tailpipe emissions directly. Other states use a combination of both. The test you take depends on where you live and what your car's model year is — older cars often face stricter testing than newer ones.

Key Takeaways

  • The 16-minute smog test measures tailpipe emissions on a dynamometer and takes about 16 minutes to complete, simulating both city and highway driving.
  • Your car passes if nitrogen oxides, particulate matter, and other pollutants stay below your state's legal limits; failure means you cannot renew your registration.
  • Not all states use the 16-minute test — some use the OBD test instead, which reads your car's onboard computer, and requirements vary by model year and vehicle type.
  • A failed test usually means you need repairs before you can retest, and repair costs depend on what the test found.

How the dynamometer test works step by step

When you arrive at a smog testing station, the technician will drive your car onto the dynamometer — a platform with rollers underneath the wheels. Your car stays in place while the rollers spin to simulate driving. The technician connects a probe to your tailpipe that measures emissions in real time.

The test follows a set pattern. Your car idles for a few seconds, then the rollers accelerate to mimic city driving (speeds up to 25 mph with frequent stops), then highway driving (speeds up to 55 mph with steady acceleration). Throughout, a computer records the levels of carbon monoxide, hydrocarbons, nitrogen oxides, and particulate matter. The whole cycle takes about 16 minutes. At the end, you get a printout showing whether you passed or failed and what your emissions levels were.

If you pass, the test station sends your results to your state's environmental agency, and you can renew your vehicle registration. If you fail, you receive a detailed report showing which pollutants exceeded the limit. You then have a set window — usually 30 to 60 days depending on your state — to get repairs and retest.

Why states require the 16-minute test

States that have not met federal air quality standards are required by the Clean Air Act to run emissions testing programs. The goal is to keep older, high-polluting vehicles off the road or force their owners to repair them. A single car that fails the test can emit as much pollution as dozens of well-maintained cars, so catching and fixing those vehicles reduces smog and ground-level ozone in populated areas.

The 16-minute test is more thorough than the OBD test because it actually measures what comes out of your tailpipe rather than just checking whether your car's computer reports any problems. This catches cars with faulty emissions systems that might pass an OBD scan but still pollute heavily. However, the test is also more expensive to administer, which is why some states have moved to OBD-only testing for newer vehicles.

Which states use the 16-minute test and when you need it

States in or near areas with poor air quality — including California, New York, Massachusetts, Connecticut, Delaware, Maryland, New Jersey, Pennsylvania, Rhode Island, Vermont, and parts of Texas — require some form of emissions testing. However, not all of these states use the 16-minute dynamometer test for all vehicles. California, for example, uses the 16-minute test for older cars and the OBD test for newer ones. New York uses OBD testing for most vehicles but may require the dynamometer test if your car fails the OBD scan.

Your state's Department of Environmental Quality or Department of Motor Vehicles website will tell you whether your specific vehicle needs testing and which test applies. Generally, the older your car, the more likely you are to face the 16-minute test. Vehicles from the 1970s and 1980s almost always require it; vehicles from the 2000s onward often may have access to for OBD testing instead.

What happens if you fail the 16-minute test

A failed test means your car's emissions exceed your state's limits in one or more categories. The test report will specify which pollutants were too high — for example, "nitrogen oxides 1.2 times the standard" or "particulate matter above limit." This information tells a mechanic where to look for problems.

Common reasons for failure include a faulty oxygen sensor, a clogged catalytic converter, worn spark plugs, a malfunctioning fuel injector, or an engine control computer that is not working correctly. Some repairs are inexpensive (replacing spark plugs or an oxygen sensor might cost $100 to $300), while others are costly (a catalytic converter replacement can run $500 to $2,500 or more). After repairs, you can retest, usually at a reduced fee.

If you cannot afford repairs, some states offer repair information programs or waivers for low-income vehicle owners. Your state's environmental agency website will list these options. A few states also allow you to retire an old, heavily polluting vehicle and receive a credit toward a newer one, though this is less common.

The difference between the 16-minute test and the OBD test

The OBD test plugs a scanner into your car's onboard diagnostic port — a connector under the dashboard — and reads data from your car's computer. It checks whether your emissions system is working and whether any fault codes are stored. It takes about 10 minutes and costs less than the dynamometer test. However, it does not measure actual tailpipe emissions, so a car with a malfunctioning sensor might pass the OBD test even if it is polluting heavily.

The 16-minute dynamometer test measures real emissions, so it catches problems the OBD test might miss. It is more expensive and takes longer, but it is more accurate. Many states use OBD testing as a first step and require the dynamometer test only if a car fails the OBD scan or if the car is old enough to require it by default.

Your state determines which test your car needs based on its model year, type (passenger car, truck, motorcycle), and whether the state has the infrastructure to run dynamometer tests. You cannot choose which test to take — the testing station will tell you which one applies when you arrive.

How to prepare your car for the 16-minute test

Before you schedule a test, make sure your car has been driven for at least 20 minutes so the engine is fully warmed up. A cold engine produces higher emissions and is more likely to fail. Also check that your gas cap is tight — a loose or missing cap can trigger a fault code and cause failure.

If your check engine light is on, get that diagnosed before testing. The light means your car's computer has detected a problem, and you will almost certainly fail the test. A mechanic can read the fault code and tell you what needs repair. If you have recently had work done on your emissions system, make sure the repairs were completed correctly — ask the mechanic for documentation.

You do not need to do anything special to your car the day of the test. Do not add fuel additives or try to "clean" your engine — these do not help and may hurt. straightforward drive normally to the testing station with a warm engine.

Frequently Asked Questions

Can I fail the 16-minute test even if my check engine light is not on?

Yes. The check engine light only comes on when your car's computer detects a fault code, but emissions can be high for reasons the computer does not flag — for example, worn spark plugs, a dirty fuel injector, or a failing oxygen sensor that has not yet triggered a code. This is why the dynamometer test is more thorough than the OBD test.

What if I fail the test but cannot afford repairs right away?

Most states give you 30 to 60 days to repair and retest. Some states offer repair information for low-income owners through their environmental agency or through nonprofits. Check your state's Department of Environmental Quality website for programs in your area. A few states also allow you to request a waiver if repairs are too expensive, though waivers are not may provide.

Do electric cars and hybrids need the 16-minute test?

Electric cars do not produce tailpipe emissions, so they are exempt from smog testing in all states. Hybrids are usually required to test because they have a gas engine, though some states exempt newer hybrids that meet strict emissions standards. Check your state's rules for your specific vehicle.

How often do I need to retest if I fail?

You can retest as soon as you have completed repairs, but most states require you to retest within 30 to 60 days of your initial failure or you will lose your registration. The retest fee is usually lower than the initial test fee — often $10 to $20 instead of $30 to $50 — but this varies by state and testing station.

Can I test at any smog station, or do I have to go to a specific one?

In most states, you can choose any licensed smog testing station. However, some states have a network of official stations, and you must use one of them. Your state's Department of Motor Vehicles website will list approved stations near you. Prices vary slightly between stations, so you can call ahead to compare.