[duke350]

Different things trip the check engine light. On a 98 you have an OBDII system which monitors a lot of things. The light can be a result of something as minor as a broken/cracked/removed vacuum line to a bad sensor/bad reading on a sensor. Without a scantool, or at least a code reader, you are just guessing. Some things will not harm the engine, but will not allow it to pass emmisions. Some things can destroy an engine. And yes, every single code needs to be clear for the check engine light to remain off. Get the codes read and we can help you get it fixed. It may be something as small as a $20 sensor. Whatever it is, when it is fixed, your car will run better and get better mileage, and your plugs will most likely last longer. The code(s) will tell you what circuit is out of specification and will help you track it down. Chances are if your light has been on for 3 years, you have spend more on wasted gas then the part/repair necessary to make it run right. One other thing, the light may have been on for something minor, but something major may have recently happened. The check engine light cannot come on again, since it is already on. It will be on if you have 1 code or 20.
For example. Say your O2 sensor goes bad. You never change it, just let it go. It would have been under $100. Over time, the engine may send to much fuel down, making it run rich (wasted cash for gas). You let it go, over a short amount of time it will ruin the cat converter. A converter is another couple of hundred dollars. It may also ruin your muffler. Not to mention the short life span of your plugs. Fouled plugs lead to higher resistance, meaning your cap and rotor go bad sooner. And the wires. And coil. And ign. module. Now, you let it go to long, you have $600-$1000 in parts alone, just because you didn't want to get the codes read to find out a sensor that costs under a hundred bucks was bad. Its your car and your choice, but if you want to know how to get the check engine light off, you need to get the codes read to know why it came on.