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What does it mean if the "Service Engine Soon" or "Check Engine" turns on while I'm driving?
If the light comes on and stays on, the OBD system has detected a problem. Your vehicle might have a condition that wastes fuel, shortens engine life, or causes excessive air pollution. If left unaddressed, these conditions could also damage your vehicle and lead to increasingly expensive repairs. For example, OBD can identify a loose or missing gas cap (which wastes fuel and contributes to smog) or engine misfire (which can lead to severe or permanent engine damage).
Source epa.gov