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New Hampshire Department of Environmental Services

Diesel Smoke Opacity

Opacity Testing Program Tip Card"It’s the Law Tip Card" Adobe Acrobat

Opacity is the degree to which smoke blocks light, and the basis for measuring the amount of smoke coming from a diesel-powered vehicle. Poorly maintained or malfunctioning engines are sometimes the cause of excessive smoke. New Hampshire State Law establishes the following smoke opacity limits.

40% opacity limit for vehicles manufactured after 1990 40% opacity limit for vehicles manufactured after 1990.
55% opacity limit for vehicles manufactured between 1974 and 1990 55% opacity limit for vehicles manufactured between 1974 and 1990.
70% opacity limit for vehicles manufactured in 1973 or earlier 70% opacity limit for vehicles manufactured in 1973 or earlier.

State Required Opacity Testing
As required by State law (266:59-c Diesel Emission Testing), the Department of Safety (DOS) conducts random roadside diesel opacity testing. Motor vehicle enforcement officers from DOS conduct the test using the Snap Acceleration Test Method (also known as the "snap test" or SAE J1667) during safety inspection stops at tolls, scales and other roadside areas. DOS officers also respond to citizen complaints regarding smoking trucks.

During the "snap test," a probe is placed in the truck’s exhaust and the driver snaps the accelerator several times while the truck is in neutral. Test results are based on opacity levels and vehicle age. An engine that is not emitting any smoke, or emits very little smoke, is likely operating efficiently and will pass the opacity test (opacity level less than 20%).

Offenses within a consecutive 12-month period Fine
First violation $ 100
Second violation 300
Third violation 500

For more information on opacity testing, visit the New Hampshire Department of Safety Web site.

Photo, failed test

How can a failed test be avoided?
Any owner/operator of a vehicle that has obvious smoke problems should take steps now to repair problems. Most such repairs are minor engine adjustments that may cost less than a first fine. Poorly maintained or malfunctioning engines and engines that have been tampered with emit excessive smoke.

Some common maintenance problems are

  • Clogged, worn, mismatched injectors
  • Misadjusted injection timing
  • Clogged or worn fuel filters
  • Restricted air filters
  • Defective turbocharger or blower
  • Bad fuel
  • Defective puff limiter or aneroid valve

Some possible repair strategies (by qualified technicians) are

  • Adjust air-fuel ratio
  • Clean or replace air filter
  • Adjust or repair turbocharger
  • Adjust or replace throttle delay and/or puff liner
  • Examine governor, fuel rack and injector timing for possible adjustment
  • Clean injector nozzles

Contact NHDES for further information or to request a free opacity test.

For more information
New Hampshire Opacity law



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