Knock Sensor
A sensor that detects engine knock (pre-ignition) and signals the computer to adjust ignition timing.
The knock sensor is an electronic sensor bolted directly to the engine block that acts as a microphone, listening for the specific high-frequency vibrations caused by engine knocking or pinging (also known as pre-ignition or detonation).
Detonation occurs when the air-fuel mixture ignites prematurely or unevenly in the cylinder, producing a sharp pressure wave that can damage pistons, spark plugs, and cylinder heads. The knock sensor utilizes a **piezoelectric crystal** that generates a small electrical voltage when vibrating. If the sensor detects knocking, it sends a signal to the engine control module (ECM).
The ECM responds by retarding the ignition timing (firing the spark plug slightly later) until the knocking stops. This allows the engine to run safely at the limit of its efficiency and accommodate different fuel octanes without damage. Modern engines typically feature one or two knock sensors located between the cylinder banks.
If a knock sensor fails, the ECM will store a diagnostic code (such as P0325) and illuminate the check engine light. To protect the engine from potential damage, the ECM will typically enter a fail-safe mode, retarding ignition timing permanently, which results in a noticeable loss of engine power and reduced fuel economy.