Today I am testing a knock sensor given a check engine light and a code for my Subaru. You can search online for your specific knock sensor voltage however this 1997 Subaru Legacy needs to be between 4.5 - 5.5 volts. Set your multimeter to DVA 20 and then place one lead on the metal part of the knock sensor where the bolt goes through and the other lead on the end of the electrical plug. The voltage should be close to your vehicle's reading from online (in my case it should be near 5). If the voltage is not close (near 0 in my case), you have a bad knock sensor.