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About the Program: Why & What We Test

Why do we test?

Emissions: Vehicle engines and air pollution

What is a vehicle test or inspection?

What happens when I bring my vehicle for inspection?
Emissions tests
Emissions testing of heavy duty vehicles
Safety tests
At the end of the inspection
If my vehicle failed, what should I do?

Why do we test?

Emissions: Vehicle engines and air pollution

Photograph of empty highway on a sunny day. Most of the 4.6 million cars, trucks, buses and SUVs registered in Massachusetts are powered by internal combustion engines that run on petroleum-based fossil fuels:

  • Gasoline, which generally contains an additive such as ethanol, evaporates easily and is highly flammable. In a gasoline engine, a spark ignites the fuel, but the combustion process is inefficient. About one-quarter of the fuel actually powers the vehicle forward; the rest is consumed by friction and heat, or emitted as exhaust.

  • Diesel fuel is heavier, oilier, less evaporative and less flammable than gasoline. In a diesel engine, the fuel is ignited through compression of air in the cylinders. Because diesel fuel has a higher energy density than gasoline, vehicles that run on diesel tend to deliver far better fuel economy than gasoline vehicles.

Both gasoline and diesel fuel contain mixtures of hydrocarbons, which are compounds that contain both hydrogen and carbon atoms. In a “perfect” engine, oxygen from the air would convert all of the fuel’s hydrogen to water, and carbon to carbon dioxide. But no combustion process is perfect, so both gasoline and diesel vehicles are equipped with emission control systems that reduce (but do not eliminate) pollutants that can be harmful to the environment or public health:

  • Nitrogen Oxides (NOx) and Hydrocarbons (HC) react in the presence of sunlight to form ground-level ozone or "smog," which can irritate eyes, damage lungs and aggravate respiratory problems. Symptoms include coughing, shortness of breath and difficulty breathing. Hydrocarbons also contribute to global climate change and many are considered hazardous air pollutants.

  • Carbon Monoxide (CO) reduces the flow of oxygen in the bloodstream and is of particular concern to people with heart disease.

  • Carbon Dioxide (CO2) is a “greenhouse gas” that traps the earth’s heat and contributes to global climate change. Due to differences in their comparative fuel efficiency, diesel vehicles tend to emit less CO2 per mile than vehicles with gasoline engines.

  • Sulfur Dioxide (SO2) contributes to the formation of acid rain.

  • Hazardous Air Pollutants (HAPs) are substances that the U.S. Environmental Protection Agency (EPA) considers dangerous or toxic. Motor vehicles can emit up to 40 of these pollutants, including 15 that are known or suspected to cause cancer. Of greatest concern are acetaldehyde, benzene, 1,3-butadiene, formaldehyde, and polycyclic aromatic hydrocarbons (PAHs). Learn more about air toxics from motor vehicles.

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In addition to these emissions (which are common to both types of internal combustion engines), diesel vehicles also produce significant amounts of particulate matter, or soot. More than 90 percent of the particulates found in diesel exhaust are fine particles, measuring 2.5 microns or less. These solid particles and liquid droplets are so small that they can be inhaled deep into the lungs and cause health problems. Large concentrations of fine particles can be seen as haze, dust, and/or soot, reducing outdoor visibility by as much as 70 percent from natural conditions. Ozone and airborne particles and droplets can remain suspended in the air for extended periods of time and can travel long distances. When they eventually settle to the surface, they can damage property, plants and animals.

Recent studies have shown links between particulate matter and premature death from respiratory and cardiovascular disease, and greater likelihood of respiratory illnesses, particularly among children and the elderly. For adults with heart or lung conditions, exposure to fine particles can cause more illness and in some cases premature death.

Greenhouse Gases & Climate Change
There is scientific consensus that our climate is changing, largely as a result of the combustion of fossil fuels, which produces carbon dioxide (CO2) and other “greenhouse gases” that form a “blanket” of pollution, trapping heat in our atmosphere and causing climate instability that can lead to severe storms, droughts, floods, heat waves and rising sea levels.

In the United States, transportation is the largest contributor to the problem, accounting for one-third of all CO2 emissions from fossil fuel combustion. More than 60 percent of these emissions come from gasoline-powered private passenger vehicles; the rest from diesel vehicles and construction equipment, aviation, shipping and other transportation activities.

The National Ambient Air Quality Standards (adopted by the U.S. Environmental Protection Agency) establish maximum levels for ozone, particles, and other air pollutants. Massachusetts’ air does not meet the standard for ground level ozone, and we are required by the U.S. Clean Air Act to implement programs that will reduce ozone levels so that our air meets the standard. One important component of our efforts is the Massachusetts Vehicle Check Program, which helps to ensure that vehicles run as cleanly as they were designed to run throughout their “life.” Since its introduction, the Massachusetts vehicle testing program has helped to reduce air pollution across the Commonwealth. Annual emission tests (which replaced tests every two years in October 2008) mean that vehicles with faulty emissions control systems are identified and repaired more quickly.

You can do your part to improve air quality in Massachusetts by maintaining your car on a regular basis— have the engine tuned and the oil changed according to the manufacturer’s guidelines, watch for tell-tale signs of pollution such as your “Check Engine” light turning on, or black or bluish smoke coming from the tailpipe, and take note of a sudden decrease in your vehicle’s gas mileage.

To learn more about the quality of Massachusetts air and state programs to improve it, please visit this MassDEP website.

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Safety is an equally important part of the Massachusetts Vehicle Check. A broken taillight, cracked windshield or a defective horn might not seem important, but any one of these or many other conditions can make driving unsafe. That’s why it’s important to make sure your vehicle is always in good repair, not just when you’re going to have it inspected. The safety inspection looks for specific criteria.

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What happens when I bring my vehicle for inspection?

First, a state-licensed inspector will give your vehicle a brief visual inspection to make sure there are no conditions such as leaking gasoline that present an immediate danger to either the inspector or the general public. These conditions must be fixed before the inspection proceeds.

After passing the visual inspection, you then present your vehicle registration to the inspector. A vehicle must have an active registration and matching valid vehicle ID number (found in the left front side of the dashboard) to get an inspection. After the inspector checks that your registration is valid, you pay the inspector $35. Drivers are then asked to exit the vehicle and remain in the waiting area while the inspector administers the safety and emissions tests.

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Emissions Tests

The On-Board Diagnostics (OBD) Test All vehicles that are registered in Massachusetts must receive a safety inspection each year. Vehicles listed below must also receive the following types of emissions tests each year:

On-Board Diagnostic (OBD) Test:

  • Model year 2001 and newer passenger cars, trucks and SUVs

  • Model year 2001 and newer light-duty diesel vehicles (with a Gross Vehicle Weight Rating or "GVWR" of 8,500 pounds or less)

  • Model year 2007 and newer medium-duty diesel vehicles (with a GVWR of 8,501 to 14,000 pounds)

  • Model year 2008 and newer medium-duty non-diesel vehicles (with a GVWR of 8,501 to 14,000 pounds)

Opacity Test:

  • Medium- and heavy-duty diesel vehicles (with a GVWR of 10,000 pounds or greater) not equipped with OBD systems

The Massachusetts Vehicle Check on-board diagnostic (OBD) emissions test is designed to ensure that your vehicle keeps running as cleanly as it was designed to run, which in turn protects the air we breathe.

The OBD test typically takes about three minutes. The inspector connects your vehicle's on-board computer to an analyzer in the station, and then downloads engine and emissions control data. The analyzer checks several OBD system functions:

Communication. Does your vehicle’s OBD system communicate with the analyzer? If your vehicle’s OBD system cannot communicate with the station’s analyzer, the OBD system must be repaired before the emissions test can be completed.

Readiness. Is your vehicle’s OBD system “ready” to be tested? As your vehicle drives, the OBD system checks the performance of various emissions-related components and systems. If the OBD system has not performed enough of these self-checks, your vehicle is “not ready” for an emissions test.

To pass the emissions test:

  • 2001 and newer model year vehicles may have a maximum of one (1) “not ready” non-continuous monitor.
  • If the vehicle failed the emissions test with a catalytic converter-related diagnostic trouble code, the vehicle’s catalyst monitor must be “ready” to pass the retest.

Diagnostic Trouble Codes (DTCs). Why would the OBD system turn on the Check Engine light? These indicators are diagnostic trouble codes that indicate which systems or components are not performing as designed. Reviewing these codes is the first step in diagnosing an emissions-related problem. These codes, along with other information in the OBD system, help guide emissions repair technicians to faulty parts and take the “guess-work” out of the process.

Check Engine Light. Is the Check Engine light (sometimes labeled as “Service Engine Soon”) turned on? When this light is turned on, it indicates that one or more components of your vehicle’s emission control system is not working as it was designed to work, and repairs are needed. If the light does not turn on when the OBD system tries to turn it on, this problem must be corrected.

The results are printed on the Vehicle Inspection Report (VIR), which the inspector will give you when the inspection is finished.

If your vehicle passes both its OBD emissions test and its safety inspection, it is issued a new sticker. If OBD detects a problem with your vehicle (generally indicated in advance by an illuminated "Check Engine" or "Service Soon" light), your vehicle will fail its inspection and will need to be repaired.

The most common causes of emissions test failures include:

  • Malfunctioning components that regulate fuel/air ratio, such as oxygen sensors

  • Exhaust gas recirculation (EGR) valves

  • Engine misfire

  • Catalytic converters

  • Evaporative controls, including poor-fitting gas caps

Sometimes, a vehicle will fail or be turned away from inspection because its OBD system is "not ready." This simply means that the OBD system did not have enough valid data to evaluate the vehicle’s emissions control system. This may be because the vehicle's battery was disconnected recently, perhaps while repairs were being made to the alternator, starter, electrical system, engine or transmission. Usually, a week of combined highway and city driving will reset the OBD system so that it will be ready for testing.

The VIR provides information that a repair technician can use to diagnose your vehicle's problem, fix it before it causes more air pollution, and spare you from more expensive repairs down the road.

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Emissions Testing of Heavy Duty Diesel Vehicles

"Snap acceleration opacity" tests are used for diesel trucks, buses and other heavy-duty vehicles (with a GVWR of 10,000 pounds or greater) not equipped with OBD systems.

In this test, the inspector uses an opacity meter or “smoke meter” to measure the smoke from the vehicle’s exhaust pipe. The darker the smoke, the more the vehicle is polluting and the higher its opacity reading will be.

The inspector first secures the vehicle safely (so it cannot move) and tests to ensure that its engine governor is functioning properly. Then the inspector presses on the throttle to bring the engine up to its maximum governed revolutions per minute (RPM) several times - first to remove loose soot from the exhaust pipe, then to measure the opacity of the vehicle’s emissions.

Readings from the final three acceleration “snaps” are averaged. The final average is compared to the emission standard for the model year and type of vehicle. Newer vehicles have more sophisticated emission controls, and must meet stricter standards.

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Safety Tests

Safety inspections typically take about 12 minutes. In a safety test, the inspector looks for or tests 14 key areas:

1. Visual Overview

  • Certificate of registration
  • Vehicle Identification Number (VIN)
  • License plate(s) Decal

2. Brake Tests

  • Parking brake
  • Service brake

3. Exhaust System

  • Exhaust system components/Muffler
  • Visible blue or black smoke

4. Steering and Suspension

  • Steering wheel and box
  • Suspension/front end
  • Springs
  • Shocks

5. Horn

  • Sound horn to test for adequate signal
  • The horn must be securely fastened to the vehicle

6. Glazing, Glass and Windshield Wipers

  • Windshield
  • Windshield wipers and washer
  • Windows
  • Tint

7. Rear View Mirror

  • Rear view mirror
  • Mirrors (General)

8. Lighting Devices

  • Head light aim
  • Hazard lights/directionals/stop/reverse lights/and license plate light(s)
  • Reflectors

9. Tires and Wheels

  • Tires
  • Wheels

10. Bumper, Fenders, and Fuel Tank

  • Bumpers
  • Fenders
  • Floor pans
  • Fuel tank
  • Vehicle Frame or Unibody

11. Altered Vehicle Height

12. Seat Belts

13. Airbags

14. Fuel Tank Cap

For a more complete list of questions and answers regarding annual safety inspections, go to the Frequently Asked Questions (FAQs) section at the Registry of Motor Vehicle’s website.

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At the end of the inspection

When the inspection is over, the inspection computer will print a sticker for your vehicle (which the inspector will place on your windshield) and a printout of the test results (your “Vehicle Inspection Report”). Then the inspector will drive your car out of the bay.

There are occasions when a motorist disagrees with the results of the vehicle inspection. The RMV has instituted a process to challenge these inspections. Read more.

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If my vehicle failed, what should I do?

Find out here.

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