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 knock sensor detects abnormal vibration (spark knocking) in the engine. The knock sensor is mounted in the engine block near the cylinders and produce an AC signal under all engine operating conditions. The powertrain control module (PCM) contains integrated knock sensor (KS) diagnostic circuitry which uses the input signals from the knock sensors to detect engine detonation. This allows the PCM to retard ignition control (IC) spark timing based on the amplitude and frequency of the KS signal being received.

The PCM calculates an average voltage the knock sensor signal and takes instantaneous signal voltage readings. The PCM uses the instantaneous signal voltage reading to determine the state of the knock sensor circuitry. If the knock sensor system is operating normally, the PCM should monitor instantaneous KS signal voltage readings varying outside a voltage range above and below the calculated average voltage. The following diagnostic trouble codes (DTCs) are used to diagnose the knock sensor system: