Knock Sensor
A piezo sensor that lets the ECU hear detonation and pull timing to protect the engine.
A knock sensor is a piezo "microphone" bolted to the engine block that listens for the high-frequency rattle of pre-ignition (detonation). The ECU uses its signal to detect knock and protect the engine.
Where it is used
Knock sensors are fitted to DOHC VTEC engines such as the B16, B18C and H22, and to SOHC VTEC OBD2 engines.
How it works
The sensor feeds a dedicated knock-detection circuit (the "knock board") inside the ECU, which watches for the engine's detonation frequency. When it hears knock, the ECU can retard ignition timing to protect the engine.
Limitations on boosted setups
The factory knock board is widely considered unreliable for detecting knock on forced-induction engines. In practice it acts as a basic safeguard against bad fuel rather than a precise knock-detection system, and it can report false positives on forged internals.
Tip: On a boosted build, the stock sensor is usually paired with a dedicated aftermarket knock-detection device (for example a J&S Safeguard) for accurate protection.