2002 Subaru Impreza Knock Sensor

The knock sensor in my vehicle intermittently comes on and off. I took it to a mechanic to have it tested. He said that it passed but they sometimes still need to be changed. I am capable of changing this myself. I am unsure of his qualifications. When asked what else it might be he was vague and suggested several expensive possible remedies. Should I change the sensor out? And if its not the sensor what else could it be? The vehicle also seems to sputter when the check engine light comes on.