When testing a knock sensor with a multimeter, you should expect to see voltage spikes ranging from 0-4.5 volts. The voltage will be determined by the condition and type of knock sensor being tested. If the readings are outside of this range, it could mean that the sensor is broken or needs to be replaced. Specific testing instructions and expected voltage ranges for your specific make and model must be found in your vehicle's service manual. Furthermore, suppose the reading does not change when you knock on the engine block. In that case, there could be a problem with the wiring harness or the connection to the ECU (Engine Control Unit) that needs to be looked into further.