Knock Sensor Code

[BigBlueSVX]

I recently replaced both of my oxygen sensors and the car has been running much better, and my fuel mileage has also improved. The problem is that the ECU is throwing a 28, refering to one of the knock sensors. The check engine light comes on randomly and may only stay on for a few seconds. The car drives and runs great as far as I can tell. Should I go ahead and get the new knock sensors? I know to replace them in pairs, and that they should run about $30. What are symptoms of a bad knock sensor?