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The check engine light is not mileage based. The check engine light comes on when the on-board computer detects a fault in engine driveability or performance, transmission control system or transmission, or the emissions system, and a fault code is set and stored in the computer. Faults that can set the check engine light can range from a vaccuum leak, engine mis-fire, low engine compression, faulty sensor such as O2 sensor or other engine sensor, hydraulic pressure loss in transmission, transmission shift solenoid, open/short in wiring harness, emission evaporative system leak, or one of over a few hundred different reasons.
If you are in doubt as to the diagnosis of what is responsible for the check engine light coming on, any reputable independent shop can retreive the diagnostic trouble codes with a diagnostic scanner and let you know if the first diagnosis was accurate. They can also give you a quote for them doing the repair, which may be significantly cheaper as their labor rates are usually lower than a dealership's hourly rates.