Spark Knock sensors is a knocking, rattling or pinging noise. It could be heard when he engine is accelerating or is working hard under load. Spark knock means the fuel is detonating. Detonation is when the fuel explodes erratically instead of burning smoothly.
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The knock sensor is located on the engine block, cylinder head or intake manifold. This is because its function is to sense vibrations caused by engine knock or detonation. The PCM uses this signal to alter the ignition timing and prevent detonation. It will compare this information with its preset tables to identify a valid knock or ping. If a ping is sensed, it will retard the ignition timing to protect the engine from this damaging pre-ignition.