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

The knock sensors detect abnormal vibration (spark knocking) in the engine. The knock sensors are mounted in the engine block near the cylinders. The knock sensors produce an AC signal voltage under all engine operating conditions. During engine operation, the PCM calculates the average voltage of each knock sensor's signal. If the knock sensor system is operating normally, the PCM should monitor KS signal voltage varying over 0.5 volt above and below the calculated average voltage.

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

DTCs P0327 and P0332 are designed to diagnose the knock sensors, and related wiring, so problems encountered with the KS system should set a DTC. However, if no DTC was set but the KS system is suspect because detonation was the customer's complaint, refer to Detonation/Spark Knock .