The Knock Sensor detects abnormal vibration (spark knock or pinging) in the engine. The knock control system is designed to reduce spark knock during periods of heavy detonation. This allows the engine to use maximum spark advance to improve driveability. Knock sensors produce an AC output voltage which increases with the severity of the knock. The signal is fed into the PCM and the timing is retarded to compensate for the severe detonation. On four-cylinder models, the Knock Sensor is located on the side of the engine block below the intake manifold. On V6 models, the Knock Sensor is located in the middle of the engine block underneath the intake manifold. To check, disconnect the electrical connector from the Knock Sensor. Using an ohmmeter, measure the resistance between the Knock Sensor terminal no. 2 (Pathfinder) or terminal no. 1 (Frontier/Xterra) and a good engine ground point. The resistance should be 500 to 620 K-ohms at 77-degrees F (25-degrees C). If the resistance is not as specified, replace the Knock Sensor. Note: The Knock Sensor resistance is very high, so use an ohmmeter capable of measuring at least 10 meg-ohms. To replace, on V6 models, remove the intake manifold. Disconnect the electrical connector from the Knock Sensor. Remove the sensor retaining bolt and remove the sensor from the engine block. Installation is the reverse of removal. On V6 models, install the intake manifold.