Varying octane levels in today's gasoline may cause detonation in some engines. Detonation is caused by an uncontrolled explosion, or burn, in the combustion chamber. This uncontrolled explosion could produce a flame front opposite that of the normal flame front that is produced by the spark plug. The rattling sound normally associated with detonation is the result of 2 or more opposing pressures, or flame fronts, colliding within the combustion chamber. Light detonation is sometimes considered normal, but heavy detonation could result in engine damage. In order to control the spark knock, a Knock Sensor (KS) system is used. This system retards the spark timing when the engine detects a spark knock. The KS system allows the engine to use the maximum spark advance for optimal driveability and for fuel economy.

The knock sensors detect abnormal vibration in the engine. The knock sensors are mounted in the engine block near the cylinders and produce an AC signal during all engine operating conditions. The PCM contains integrated knock sensor (KS) diagnostic circuitry which uses the input signals from the knock sensors in order to detect engine detonation. This allows the PCM to retard Ignition Control (IC) spark timing based on the amplitude and the frequency of the KS signal that it receives.