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In order to determine why your Check Engine Light is on, a mechanic will connect a handheld device to the OBD II connector under your dash (in a vehicle made in 1996 and onward).

All cars made from 1996 onward have an onboard computer that detects faults in the engine, transmission and emissions systems, and communicates problems through lights in the dash (your Check Engine Light, for instance).

Most cars today (anything manufactured after 1996, actually) have an onboard computer that stores trouble codes when something goes wrong with a major component in the engine, transmission or emissions system.