Varying octane levels in todays gasoline may cause detonation in some engines. Detonation is caused by an uncontrolled explosion in the combustion chamber. This uncontrolled explosion could produce a flame front opposite that of the normal flame front produced by the spark plug. The rattling sound normally associated with detonation is the result of 2 or more opposing pressures colliding within the combustion chamber. Though light detonation is sometimes considered normal, heavy detonation could result in engine damage. To control spark knock, a knock sensor (KS) system is used. This system is designed to retard the spark timing when a spark knock is detected in the engine. The KS system allows the engine to use the maximum spark advance for optimal driveability and fuel economy.

The PCM contains integrated knock sensor (KS) diagnostic circuitry. The input signal from the knock sensor is used to detect engine detonation, allowing the PCM to retard the ignition control (IC) spark timing based on the amplitude and frequency of the KS signal being received.

The knock sensor detects abnormal vibration or spark knocking in the engine. The knock sensor is mounted in the engine block. The knock sensor produces an AC signal voltage under all engine operating conditions. During engine operation, the PCM calculates the average voltage of the knock sensor signal. If the knock sensor system is operating normally, the PCM should monitor a KS signal voltage varying over 0.5 volt more than and less than the calculated average voltage.

If the PCM malfunctions in a manner which will not allow a proper diagnosis of the KS circuits, DTC a P0325 will set.

A DTC P0327 applies to the knock sensor and the related wiring. However, if no DTC was set but the KS system is suspected because detonation was the customers complaint, refer to Detonation/Spark Knock .