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A code 21 indicates a bad O2 sensor aka oxygen sensor.

What oxygen sensors are:
The O2 sensor mesures the level of oxygen left in the exhuast. Mesuring this is a good way to tell what the air/fuel ratio is. If there is a lot of oxygen, then you are runing lean.The ECU looks at the mesurement of this sensor, and a few others and makes corrections to the fuel curves if need be.

Effects of a bad sensor:
Your car will usually run rich. Therefor your gas mileage will decrease. That is all. No engine damage will result.

If you have a 93-95 car, you have 2 (ODB-1). If you have a 96-97 car, you have 4 (ODB-4).

On 96-97 cars, the upstream sensors are usually the ones that fail. The downstreams are merely to make sure the cat is doing its job.

Replacing the sensors is fairly easy. You just need to unscrew the old sensors, and screw in the new ones. You can either use an open end wrench, or a special socket designed for oxygen sensors.

Here is the how to on replacing them:
http://www.dodgeintrepid.net/forums/showthread.php?s=&threadid=24812
 
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