Vehicle Testing

OBD II – on-board diagnostics

The Clean Air Act required automakers to include On Board Diagnostics (OBD) in all cars, vans, and light-duty trucks beginning in the model year 1996.  OBD equipment tests vehicles equipped with OBD technology.  OBD II checks for engine performance and can detect problems with the emissions control system before the driver is aware of them.  The system monitors for malfunction or deterioration of the power train and its emissions-control systems on a constant basis.

The OBD II test is conducted by attaching a cable to the vehicle’s on-board computer through a data link connector (DLC) usually found under the dashboard, and information is downloaded to a computer regarding the vehicle’s emissions systems.

Two-speed idle test

The vehicle idles for 30 seconds, is then accelerated to 2500 revolutions per minute for 30 seconds, and then idles for 30 seconds.  An air sampling probe, placed in the tailpipe, collects information on the vehicle’s emission of hydrocarbons and carbon monoxide.  The single- or two-speed idle test is part of a basic emissions inspection program used in moderate or marginal non-attainment ozone areas.  This test is conducted with a four-gas analyzer.

Acceleration simulation mode (ASM) or loaded-mode

ASM testing requires a five-gas analyzer and dynamometer (a treadmill-like device that spins the front or rear tires of the vehicle). During the ASM test, the lane inspector inserts a probe into the tailpipe to collect and analyze exhaust that is emitted while the vehicle is accelerated and decelerated in the range of 0-30 miles per hour during the enhanced (or loaded mode) test.  This procedure more accurately measures vehicle emissions by simulating actual driving conditions (in other words, while the car is under load).  The test measures levels of hydrocarbon, carbon monoxide and nitrogen oxides.  It is mandated in serious, severe or extreme ozone non-attainment areas with urbanized populations over 200,000.

Gas cap pressure

A lane inspector places and then pressurizes an adapter on the vehicle’s gas cap.  The test procedure measures leak flow rate to determine whether the gas cap seal is sufficient to prevent gasoline from evaporating into the air from the vehicle’s tank.  A computer sets the target pressure for each test.  The computer also selects the correct gas cap test.

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