Most vehicles operating within allowable limits are identified by this visual check, and are not put through a full test. You can do your own visual test of your vehicle's emissions here . If inspectors decide a vehicle may be emitting excess pollutants, they may direct the vehicle to a safe spot for a full emissions test. It is also used in air-quality programs in Ontario and the United States. AirCare ON-ROAD inspectors use the J1667 Snap Acceleration Test to measure the opacity of diesel emissions.
Opacity is a measurement, on a percentage scale, of how much light is blocked by the exhaust plume.
Light smoke has a lower opacity than dark smoke. A low opacity reading is good for your vehicle and good for the environment. An inspector places a smoke-sensing probe at the end of the exhaust pipe. With the vehicle in neutral, the driver quickly presses the accelerator and holds it for several seconds before returning the engine to idle. This is done several times to clear soot build-up in the system prior to the test, and to ensure the vehicle's engine is in proper mechanical condition.