Varying octane levels in today's gasoline may cause detonation in some engines. Detonation is caused by an uncontrolled explosion (burn) 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 two or more opposing pressures (flame fronts) 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 spark timing when spark knock is detected in the engine. The KS system allows the engine to use maximum spark advance for optimal driveability and fuel economy.

The PCM uses a knock sensor (KS) to detect abnormal vibration in the engine (detonation/spark knocking). Mounted in the engine block, under the intake manifold, the knock sensor produces an AC signal at all engine speeds and loads. The PCM then adjusts the spark timing based on the amplitude and frequency of the KS signal.

A knock sensor module is no longer used to diagnose the knock sensor system. The circuitry is integrated into the PCM. The PCM uses the knock sensor to determine the amount of normal engine noise (noise channel) for a wide range of engine speeds and loads. The PCM compares the actual knock sensor signal to the learned noise channel.

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

If the PCM detects an excessive engine knock and is unable to eliminate the knock, DTC P0326 will set. This DTC indicates that the vehicle is operating with excessive amount(s) of spark retard.

DTC P0327 indicates that the PCM detects a noise channel below the learned value for the given engine speed and load. Problems encountered with the KS system should set a DTC. However, refer to Detonation/Spark Knock in Symptoms if no DTC sets, and the customer's complaint of detonation points to the KS system.