Three basic levels of equipment involved in emissions testing:
- Older testing equipment measures emission while the car is idling in Park or Neutral. The tailpipe is hooked up to a machine that captures the emissions the car produces and measures the levels of the pollutants produced.
- Newer tests, sometimes called the IM/240 test, are performed using a dynamometer, or “dyno.” Dynos are kind of like treadmills for cars; they allow the test operator to sit inside the car and “drive” it on a set of rollers that keep the car in one place but allow it to accelerate up to about 57 miles per hour. Like the older version of the test, the tailpipe is hooked up to a machine that captures and measures the emissions produced by the vehicle. This is a more accurate test, because it measures the emissions actually produced when the car is being driven, not just when the car is idling.
- The latest tests use the vehicle’s on-board computer, called an “On-Board Diagnostics System” or OBD. Cars that have an OBD system get plugged into and scanned by the testing device. The original OBD systems were installed on cars during model years 1986-1995 in the United States. Vehicles manufactured since 1996 have a more sophisticated second-generation OBD system called OBD-2.
Many testing stations also do a gas-cap test to make sure the cap seals properly to keep fuel vapors in the tank instead of releasing them into the air.