Varying octane levels in gasoline may cause detonation in performance engines. Detonation is sometimes called spark knock. A spark knock causes the pistons and the rings to vibrate and rattle, sometimes producing a knocking sound.

In order to control spark knock, a knock sensor (KS) is used. This system is designed to retard the spark timing up to 15 degrees in order to reduce spark knock in the engine. This allows the engine to use a maximum spark advance, which improves driveability and fuel economy.

The PCM uses information from the MAP sensor, the coolant sensor, and the RPM to calculate spark advance as follows:

Low MAP output or high resistance in the engine coolant temperature (ECT) sensor circuit can cause detonation. High MAP output or low resistance in the ECT circuit can cause poor performance.

The knock sensor (KS) system uses a knock sensor that is mounted into the engine block near the cylinders. The knock sensor produces an AC voltage which increases with the severity of the knock. This signal voltage inputs to the powertrain control module (PCM). The PCM then adjusts the ignition control (IC) timing to reduce spark knocking.