📖 What It Does
Allows the engine to run aggressive timing for power while still being safe on whatever octane fuel is in the tank. Without knock sensors, modern engines could not safely run today's high compression ratios.
⚙ How It Works
A piezoelectric crystal inside the sensor generates a voltage when it vibrates. The ECU filters that signal for the specific frequency of knock (usually 5–8 kHz). When knock is detected, the ECU pulls 2–5 degrees of ignition timing within a single combustion cycle.
⚠ Symptoms When It Fails
Symptoms include the P0325 or P0328 code, reduced power (the ECU pulls timing as a precaution), slightly worse fuel economy, and occasionally pinging on premium-required vehicles. Knock sensors rarely fail outright - more often it is a wiring or connector issue. See full symptom guide.
💰 Replacement Cost
Replacement: $200–$600. The sensor itself is $40–$200. Labor varies widely - some V engines need the intake manifold off ($400+ labor). See knock sensor cost.