Subaru Repair: 97 subaru legacy, subaru legacy, emmissions

Question
Check engine light comes on.  I've had it checked and they tell me it's the knock sensor light 1.  What does this light do, how does it effect my emmissions (by the way my car won't pass emmissions because of this light), can I replace it myself and how much does it cost to buy parts to fix the problem.  Fustrated with not passing emmissions any help would be greatly appreciated.

Answer
The Check Engine Light monitors the emmsion portion of the car. The knock sensor is a sensor located on top of the engine that listens for spark knock and adjusts timing as needed. When the sensor is failing to perform it's job it can't adjust the timing in turn effects how much emmisions the vehicle puts out, in turn, turns the CEL on.