97 Prelude Knock Sensor error code?

The knock sensor listens for the PING of preignition in the cylinders, usually caused by using the wrong octane gas. When it registers a PING, the EMC changes the timing a bit, until the PING is gone. If you've replaced the sensor, then maybe it's doing it's job & registering a PING. Better get it fixed, because the reason the manufacturer put it there is that bad preignition can blow the head off your car.