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Smoke opacity is a measurement of light extinction, or the blackness of the exhaust plume, expressed as a percentage. 100% opacity would be completely dark, and 0% would be totally transparent. The black smoke we see in diesel exhaust is composed primarily of carbon particles resulting from the combustion of diesel fuel.

The opacity limit is measured at the peak, or darkest point, of the exhaust cloud. This measurement is most commonly taken during the snap acceleration test, where the vehicle is at idle, with all brakes off, and the throttle is "punched" to the floorboard, initiating the exhaust cloud. Opacity is measured as a surrogate for particulate matter.

The table below illustrates the approximate opacity of our current emission standards.

opacity chart

HDDVs are subject to opacity standards based on the production year of the vehicles' engine. The standards were devised to take into account the state of engine technology, and typical wear conditions. The opacity standards are:


Current Opacity Cutpoints

Heavy-duty trucks - 18,000 pounds or more
1990 and older
1991 or 1996
1997 and newer
No visible smoke >3 consecutive seconds

Commercial & School Buses - 18,000 pounds or more
1987 and older
1988 - 1993
1994 and newer
No visible smoke >3 consecutive seconds
Retrofitted* Diesel Buses - 18,000 pounds or more
1993 and older
1994 and newer
No visible smoke >3 consecutive seconds
* Equipped with post-manufacture internal and/or external emissions control technology, and/or exhaust after-treatment devices, as required under the USEPA Urban Bus Program



Violations are placed upon owner or lessee, not the operator.
The fine for a first offense is $700, reducible to $150, with proof of repair.

For second and subsequent offenses the fine is $1300, reducible to $500 with proof of repair.

Failure to comply with the annual inspection requirement carries a fine of $500.



Established by Public Law 1995, Chapter 157, the rules that govern the Diesel Inspection Program are:

NJ Department of Environmental Protection

N.J.A.C. 7:27-14 - Control and Prohibition of Air Pollution from diesel- fueled Motor Vehicles

N.J.A.C. 7:27B-4 - Air Test Method 4: Testing procedures for Motor Vehicles

NJ Department of Transportation

N.J.A.C. 13:20-26 - Periodic Diesel Inspection Program
N.J.A.C. 13:20-46 - Diesel Emission Inspection and Maintenance Program
N.J.A.C. 13:20-47 - Diesel Private Inspection Facilities - Licensing

NJ State Police

N.J.S.A. 39:8-59 - 76 - Periodic Diesel Inspection Program


Questions and comments about the Diesel Inspection Program: