Some advocate a resistance check, but I am told that is not reliable. The knock sensors are not serviceable, but can be purchased for $40 US or less. Do you have engine knock? The knock sensor is a piezo-electric transducer. Engine knock produces a signal at about 7.5 kHz, which the knock sensor is tuned for. When knock occurs, the ECU retards the timing by 2.8 degree increments until knocking ceases to a maximum to 10 to 15 degrees (maximum retard is determined by the ECU. When knock ceases, the timing is advanced in 0.35 degree increments until knock occurs.