The On-Board Diagnostic Test
The On-board Diagnostic (ODB) test reads a computer system in your vehicle to identify engine-related problems that may result in increased emissions of harmful pollutants. If you maintain your vehicle when you are alerted by your OBD system, you can help improve the state's air quality. (For models earlier than 1996, New Jersey will continue to use the earlier tailpipe-emissions tests during inspection.)
The OBD system monitors your vehicle at all times, even when it is not being tested. If it detects something that might cause your vehicle to emit excessive harmful pollutants, the "Check Engine" light will illuminate on your dashboard. This could indicate a simple problem, such as a faulty spark plug that should be replaced before it creates a more serious problem, such as a damaged catalytic converter.