How to Replace a Nissan Xterra Knock Sensorby Alibaster Smith
The knock sensor in a Nissan Xterra performs a critical job. It "listens" to your engine to make sure that the engine is firing properly. If too much fuel gets into the engine, the compression is high enough, and the timing of the spark plugs is not precise enough, the pistons inside the engine can literally bang against the cylinder walls of the engine. This is called engine knock, and it can quickly cause catastrophic engine failure in your Xterra. When the knock sensor in your Xterra fails, you'll notice a marked decrease in fuel economy as the engine tries to retard ignition timing enough so that the engine "runs rich" (burns a lot of fuel). This will ensure that the engine does not suffer from knocking, but it also hampers fuel economy as well as engine performance.
Remove the bolt securing the knock sensor. The knock sensor is mounted behind the intake manifold on your engine, near the firewall. This is the hardest part about changing the sensor because it's a difficult location to reach. The sensor looks like a small doughnut-shaped component with a rubber top and an electrical-plug connector coming off one side of the sensor.
Remove the electrical plug by pulling it straight off the sensor and pulling the sensor out of the engine bay.
Plug the new sensor into the factory-wiring harness. Make sure you plug the sensor in before bolting it up. If you try to bolt the sensor first and you accidentally drop the sensor, it will most likely be damaged and unusable.
Align the mounting hole in the center of the sensor with the mounting hole on the engine. Bolt the sensor to the engine and tighten to 20 lb.-ft. maximum ("lb.-ft" refers to the "tightness" of the bolt and measures the force of one pound acting from a distance of one foot perpendicular to a given pivot point). Since you cannot really get a torque wrench in there for an exact torque measurement, 20 lb-ft. is just slightly tighter than hand-tight. Do not over-tighten the knock sensor or you will damage it.