OLR Research Report

November 3, 2003




By: James J. Fazzalaro, Principal Analyst

You asked several questions relating to exhaust emissions inspection requirements for trucks including:


All vehicles with gross vehicle weights above 10,000 pounds are exempt from the Connecticut emissions inspection program. All vehicles that are 25 or more years old, including trucks, are also exempt. Certain other exemptions might also apply, but in a very limited way. However, should the environmental protection commissioner determine that vehicles over 10,000 pounds need to be inspected for Connecticut to comply with federal pollution reduction requirements, the motor vehicle commissioner already has statutory authority to provide for testing.

The exemption for vehicles over 10,000 pounds is primarily a matter of state policy rather than federal mandate. The additional costs of providing equipment and facilities capable of meeting the special requirements for safely inspecting larger vehicles compared to the small benefit that would be realized from the relatively few vehicles that could be tested was the main reason for them being excluded. There are also more elaborate safety procedures that must be followed to safely operate larger vehicles in a dynamometer-based test.

Trucks up to 10,000 pounds are tested at the same inspection facilities as cars, using the same test equipment. However, diesel-powered vehicles, both cars and trucks, will only be able to go to 50 locations specifically designated to conduct diesel inspections.

Several different test procedures are used depending on the age of the vehicle, whether it is diesel-powered or not, and whether it is more than 8,500 pounds gross weight. Test procedures differ according to whether the vehicle is new enough to be equipped to accommodate new computer-based on board diagnostic test procedures, whether it fits on the dynamometer, if it has all-wheel drive, and a few other factors.

There is no simple way to do direct vehicle-to-vehicle comparison of pass-fail standards for the Connecticut system. The standards that apply depend to some extent on the test procedure being used for a particular vehicle. While it can generally be said that pass-fail standards are most stringent for newer vehicles and become increasingly less stringent for older model years, there are many variables built into the federal scheme. A particular vehicle’s test standard is most dependent on its model year and engine size, but particular vehicles of the same model year and engine size might fall into different vehicle class categories that could result in different standards.


Any truck that has a gross vehicle weight of more than 10,000 pounds is exempt from the emissions inspection program being managed by the state’s contractor, Agbar Technologies, Inc. Certain trucks under 10,000 pounds gross weight would also be exempt if they are 25 or more years old or are registered as farm vehicles as any type of vehicle in either of these categories is exempt. Certain other exemptions under the Connecticut law might apply in very limited ways. For example any vehicles (1) powered by electricity, (2) operating under temporary

registration, (3) registered but not designed primarily for highway use, or (4) operated by a licensed dealer or repairer either to or from a location of sale and an emissions inspection station are exempt. This could affect a truck-type vehicle, but this would be relatively rare.

However, the emissions inspection law provides a way for some of the exemptions, including the exemption for vehicles over 10,000 pounds to be overridden administratively. If the environmental protection commissioner finds it necessary to inspect motor vehicles that are currently exempt under the current law because they are over 10,000 pounds gross weight, motorcycles, or are four or less model years old in order to achieve compliance with federal law governing required emissions reductions, the DMV commissioner may adopt regulations to require their inspection (CGS 14-164c(c)).

Although trucks over 10,000 pounds are exempt from the Agbar-operated emissions program that applies to passenger vehicles, certain large diesel-powered vehicles with commercial registrations are subject to exhaust inspections as part of the Department of Motor Vehicles (DMV) commercial vehicle safety inspection program. The program applies to all diesel-powered commercial vehicles that are: (1) over 26,000 pounds gross vehicle weight rating, (2) designed to transport 16 or more passengers (including the driver), or (3) transport hazardous materials requiring placards under federal law (regardless of their weight).


The inspection exemption for vehicles over 10,000 pounds is primarily a matter of state policy in designing its statewide inspection program rather than a federal requirement. However, the policy has some basis in the fact that, particularly when Connecticut and other states were designing their programs in the mid 1980s, there were few emissions controls available for most heavy duty engines typically found in heavier trucks. Part of the reason was the additional cost for equipping the emissions inspection stations to handle these large vehicles. While each station was constructed with one high bay to accommodate trucks, the chassis dynamometer required for heavier vehicles was more costly than what was required for most other vehicles. Another factor was that a large portion of the bigger trucks traveling in Connecticut were not Connecticut-based trucks, that is, they are registered in other jurisdictions. Vehicles registered in other states are not subject to Connecticut’s emissions inspection law, except when the inspections of diesel vehicles are made as part of DMV’s safety inspection program.

One final factor that mitigates against inspecting these larger vehicles is the safety concerns attendant to running high horsepower vehicles on dynamometers. Special procedures, such as chaining the vehicle down, must be used when vehicles with more than 300 horsepower are run on dynamometers.


If a truck 10,000 pounds gross weight or less is powered by something other than diesel fuel (i. e. , gasoline, propane, etc. ) it will be tested at any of the dealer or repairer locations that are part of the emissions testing system being managed by Agbar. If the truck is diesel-powered, it must go to any of 50 facilities Agbar is required to equip with the capabilities to conduct diesel inspections. This also applies to any cars that are diesel-powered. Currently, 43 of the required 50 facilities have been selected. They are distributed throughout the state as follows: eight in Fairfield County; 11 in Hartford County, six in Litchfield county, three in Middlesex County, nine in New Haven County, three in New London county, two in Tolland county, and two in Windham County. The other seven sites have not yet been determined.

Heavy-duty diesel commercial vehicles subject to exhaust inspections under the DMV’s commercial vehicle inspection program are inspected wherever DMV is conducting its inspections at the time. This is usually in official truck weighing and inspection facilities, but could occur elsewhere as well.


Each of the facilities on the Agbar-managed network is equipped with the same test equipment, including a dynamometer. Trucks subject to testing requirements will be tested on the same equipment as cars, including the same dynamometer. However, not all trucks will fit on the dynamometer and therefore, these vehicles will have a different type of test. The same is true for any all-wheel drive vehicles (car or truck) because they cannot be safely tested on a dynamometer.

Cars and trucks that are capable of being tested using on-board diagnostics will be tested with the same equipment. On-board diagnostic (OBD) systems are found on newer motor vehicles. A computer module in the vehicle collects and stores data on how the various vehicle systems

are functioning. The computer at the inspection station is connected to this OBD module and information relative to its emissions performance is downloaded and evaluated. Vehicles with OBD capability will be tested in this way. Vehicles without OBD capability will be tested on the regular test equipment.


Vehicles Tested in the Statewide System

The primary test procedure that will be used for 1996 and newer non-diesel powered vehicles is the OBD II test procedure described above. The OBD procedure will also be the primary test for diesel-powered vehicles that are model year 1997 or newer and 8,500 pounds gross weigh or less. If for some reason a data communication link cannot be established for these vehicles, the back-up test procedure is the ASM25/25 procedure described below.

For gasoline-powered vehicles that are model 1995 and older, the primary test procedure will be the Acceleration Simulation Mode or “ASM 25/25” test. This procedure is designed to simulate acceleration during actual operation. The test uses a treadmill-like device called a dynamometer. The vehicle is run at 25% of the load required to accelerate at 3. 3 mph/second at a speed of 25 mph. Exhaust emissions are sampled through a probe inserted in the tail pipe.

Full time four-wheel drive vehicles and vehicles with traction control that cannot be deactivated cannot get the ASM 25/25 test because they cannot be safely operated on the dynamometer. Instead, they receive a test called the Preconditioned Two Speed Idle Test. This test procedure samples and measures the exhaust with the vehicle in neutral. The engine is run up to a speed of between 2,200 and 2,800 rpm for 90 seconds then to an idle speed of between 350 and 1,100 rpm for 90 seconds. During these periods, the exhaust gases are measured for compliance. The Two Speed Idle Test is the secondary test procedure designated for 1995 and older vehicles that cannot operate on the dynamometer for some reason. It is the primary test procedure for all non-diesel vehicles over 8,500 pounds gross weight. Many of these larger vehicles do not fit on the dynamometer.

A different test is used for diesel-powered vehicles that are over 8,500 pounds gross weight or are 8,500 pounds or less and model year 1996 or older. The test is called the 30 mph Steady State Opacity Test. The vehicle’s exhaust is measured for compliance while it is running on the dynamometer at a constant simulated speed of 30 mph and an eight horsepower loading. The vehicle exhaust cannot have an opacity of more than 20%. (Unlike non-diesel powered vehicles where the exhaust is measured for the presence of carbon monoxide and hydrocarbons in certain amounts, diesel exhaust is analyzed for its opacity, which indicates the presence of particulates, the hazardous byproduct that is being regulated. )

If these diesel-powered vehicles cannot fit or cannot use the dynamometer for some reason the secondary test that is used is called the Modified Snap Acceleration Opacity Test. The test is performed while the vehicle is in neutral. The procedure is a modified version of the procedure used on heavy duty diesel vehicles (see below). The inspector accelerates the engine sufficiently so that the engine temperature increases and the exhaust clears. An average of several tailpipe readings is taken. The pass-fail standard is 40% opacity.

Heavy Duty Diesel Commercial Vehicles

Heavy duty diesel commercial vehicles undergoing exhaust smoke tests during DMV safety inspections are given the Snap Acceleration Test. The accelerator is rapidly depressed and held at a governed speed for a few seconds and then returned to idle. This is repeated several times while the test equipment measures the opacity of the smoke. The final result is the average of the last three readings. The pass-fail opacity standard for these vehicles is 40% for 1991 and newer vehicles and 55% for 1990 and older vehicles.


As noted above, because diesel exhaust contains different regulated pollutants than those produced by combustion of gasoline, the pass-fail standards for diesel-powered vehicles are based on the opacity of the exhaust smoke. Generally speaking, all diesel-powered vehicles subject to the primary test procedure must meet a 20% opacity standard. The modify Snap Acceleration Test given to vehicles over 8,500 pounds requires them to meet a 40% opacity standard.

The pass-fail standards being used for non-diesel powered vehicles are taken from Environmental Protection Agency (EPA) guidelines for the ASM25/25 test procedure. The Connecticut program is using a recently revised set of standards developed by EPA to address a problem of false failures that were being reported by states using the previous standards.

Generally speaking, a vehicle’s pass-fail standards is primarily determined by its model year and its engine size (displacement). The data that the inspection software uses to make this placement is taken from data encoded in its vehicle identification number (VIN).

Under the standards in place for the Connecticut system, the hydrocarbon standard for light-duty trucks is as follows: 82 parts per million (ppm) for 1996 and newer vehicles, 160 ppm for 1984 to 1995 vehicles, and 340 ppm for 1983 and older vehicles. This is a straight comparison regardless of engine size. However, in all other areas (light-duty trucks for carbon monoxide and nitrogen oxide and light-duty vehicles for all three pollutants), engine size is taken into consideration. A vehicle’s engine displacement in liters is multiplied by the exhaust pollutant level and compared to the pass-fail standard. For cars, the hydrocarbon level is 300 liters ppm for 1990 and newer vehicles and 500 liters ppm for 1989 and older vehicles. For carbon monoxide, it is 1. 6 liters % for 1983 and newer vehicles and 2. 3 liters % for 1982 and older vehicles. The carbon monoxide standards for trucks are 4. 4 liters % (1996 and newer), 12. 96 liters % (1984 to 1995), and 23. 28 liters % (1983 and older).

For nitrogen oxide, the standards for cars are 3,500 liters ppm for 1981 and newer vehicles and 4,750 liters ppm for 1980 vehicles. For trucks they are 14,000 liters ppm for 1996 and newer trucks, 16,800 liters ppm for 1988 to 1995 trucks, and 32,200 liters ppm for 1987 and older trucks.

While its is very difficult to make direct vehicle-to-vehicle comparisons, it is generally true that the most stringent pass-fail standards apply to newer vehicles and they tend to become increasingly less stringent as model years increase in age. The main reason for this is that emissions control equipment has become increasingly more effective and efficient over the years.

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