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The check engine light is a warning light that comes on if the computerized engine control system detects an engine performance or emissions problem. Also called the "malfunction indicator lamp" (MIL) or "Service Engine soon". It can come on for any of hundreds of different causes from any of the systems that the computer monitors. Some are very important and some not as serious.
Your first step is to have the computer scanned to retrieve the trouble codes that were set when the check engine light came on. Those codes will get you pointed in the right direction to find your problem. With a 1996 or newer and some 95 cars and you have an AutoZone in your area, they will read them for free.
Although they can clear the codes and reset the light, if you haven't corrected the problem that put it on in the first place, it will just come back on again.
2003 Jetta left rear window blue plastic part of the