The Knock Sensor measures engine vibrations, cuts down on engine spark knock during heavy detonation, and lets the computer advance spark timing to make your car easier to drive. The sensor creates AC power that rises in strength when knocks grow tougher, and tells the PCM (powertrain control module) to step back the spark timing. Four-cylinder engines put their sensor on the block's lower side below the intake manifold, while V6 engines have it in the middle of their block under the intake. To see if the sensor is working properly, kill the power and test the resistance between terminal 2 and the engine's solid ground by using an ohmmeter. A good reading should show 500 to 620 K-ohms at 77-degrees F. When the resistance reading differs from these numbers, you need to get a new sensor. On 6-cylinder models, the intake manifold needs to come off first. Second, unplug the electrical wire attached to the sensor. Last, remove the securing bolt, then lift the sensor out of its position in the engine block. To reinstall the sensor, follow steps in reverse order.