Generally, the problem is
in the emissions or something that is affecting the effectiveness of the
emissions, but other causes can trigger the light to come on, too. To
determine the cause, the vehicle will need to be taken to AutoZone or a
shop that has an OBD2 scanner, or for 1995 and earlier models, an OBD1
scanner, While a scanner will not give you the magic answer, it will
give you a code that narrows down the problem area.Some specific causes for the light to be on are:
Gas cap left off, not tightened, not installed correctly, or needs to be replaced.
Engine misfire - error might clear itself after 3 complete warm-up cycles; not restarts, but warm-ups.
At start up for a bulb check - Should not stay on.
Details here: http://www.chinasinoy.com/service/why-would-a-check-engine-light-be-on-62.htmlCar rough idling causes
misfiring engine is among the most common causes of a rough idle. You
may notice your car idling roughly when it's in "Park" or when you're
driving it and stop at a stoplight. A rough idle can be caused by
damaged spark plugs or spark plug wires or a burned-out valve. It's
important to address the cause as soon as possible to avoid potentially
damaging the engine. Engine repairs are almost always expensive.
Learn more here:http://www.chinasinoy.com/service/car-rough-idling-causes-and-how-to-fix-it-108.html
Hope this helpful.