OBD (On-board Diagnostics)
Check Engine Light
Most 1981 and newer vehicles are equipped with some form of on-board diagnostic computer. Since 1996, all light duty vehicles come standard with the OBD II system.
For OBD II vehicles, the vehicle computer monitors a number of emissions related factors and reports any variations from the “standards”. If the vehicle is operating out of the allowable ranges, the computer turns on the “Check Engine” light on the dashboard. This light may also be displayed as “Service Engine Soon”.
1996 and newer vehicles CANNOT PASS emissions testing with the “Check Engine” light illuminated. Determining what has caused the “Check Engine” light to illuminate, and repairing the problem is a job for trained professionals equipped with the proper tools to diagnose and repair the problem. More information on the OBD system may be found in our On-Board Diagnostics Fact Sheet.
Also related to the OBD II system are “Readiness Monitors”. Monitors are not physical devices, but rather programs within the on-board computer(s) that relay to the technician if an emission device or system is not ready for testing. A vehicle will be rejected from emissions testing if: on 1996-2000 model year vehicles, more than two monitors are reporting a not ready state or more than one monitor on 2001 or newer models. For more information on Readiness Monitors, see OBD Monitors.