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Vehicle Emissions Testing

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About the Test: Understanding the Illinois Vehicle Emissions Test

What Vehicle Emissions Tests are Performed in Illinois?

An OBD test (On Board Diagnostics) is used for most 1996 and newer model year passenger cars and light duty trucks, including vans and sport utility vehicles and 2007 and newer model year heavy duty vehicles with a gross vehicle weight rating (GVWR) between 8,501 and 14,000.

How is the OBD test performed?

The OBD test consists of connecting a scan tool to a vehicle's on-board computer to determine the OBD system status (readiness checks), to determine whether the Malfunction Indicator Light (MIL) is commanded to be illuminated, and to retrieve any stored diagnostic trouble codes.

Vehicles will pass the OBD test if the OBD system is working, all required emissions system readiness checks have been completed, and the MIL is not commanded to be illuminated.

Vehicles will fail the OBD test if the MIL is commanded to be illuminated and diagnostic codes are present. Vehicles will also fail if the OBD system is inoperable or if the OBD connector is missing, damaged, or inaccessible.

Check the “Understanding On-Board Diagnostics (OBD)” frequently asked questions page for further information.

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