✔The Knock sensor is located on the engine block and in some applications under the intake manifold. ✔The Knock sensor generates an AC voltage created from the frequency of poor combustion in the engines combustion chambers. ✔A mechanical fault within the motor creating a rapping sound may create false signals from this sensor. ✔Ignition timing and the air/fuel ratio are adjusted to compensate for this condition. ✔Failure Symptoms: Malfunction Indicator Light illumination, failed emission test, pre-detonation or "ping", increased emission levels, low engine power.