Defective knock sensors can be identified by their response to engine noise or vibration. If the multimeter reads 0 volts when the engine is running, and there is no change in voltage when knocking on the engine block, it could indicate a faulty sensor. Additionally, suppose the readings taken with a multimeter are outside of your vehicle's expected range as specified in its service manual. In that case, this could also point toward a malfunctioning or worn-out sensor.