What is a Knock Sensor

The knock sensor is the unit in the car that protects against incorrect timing issues. It allows the engine to run with the timing advanced as far as possible without damaging the motor or loss of power. Unlike oxygen sensors or the air fuel ratio sensor the knock sensor will not allow the car to run incorrectly. A knock sensor feed info to the ECU and if the car is knocking too much to be adjusted by timing to will send the ECU into a safe mode which will stop the engine all together.

The knock sensor works by responding the knock caused by pre-detonation of the air/fuel mixture. When the fuel mixture in a cylinder is ignited there is a flame front that moves out from the source of the spark. When this pressure wave hits the cylinder walls it makes a noise and this is detected by the Piezoelectric element in the sensor and communicated to the ECU.

When the ECU detects these that are not at the correct time it triggers an adjustment in the motors timing. If the knock sensor fails it can cause the vehicle to run very rough or not start at all. When failure occurs a trouble code is triggered which may show as the sensor or as something else in the motor. A huge factor to remember with ECU codes is that they only show the source of the code, not the issue. In other words the code may be caused by a loose wire but it says knock sensor, it is important to remember this when diagnosing an issue. 

What Does a Knock Sensor Do?

Knock sensors are the units in vehicles that monitor the pinging in the motor and help control the timing of the engine. The computer uses the input from the knock sensor along with the O2 sensors and air fuel ratio sensor to adjust the timing and fuel mixture in the pistons.

The engine in a modern vehicle has to be kept in perfect timing to ensure the top of efficiency and performance. This is done by toeing the line between running perfect and detonation. The perfect ignition of the air / fuel mixture happens at the exact moment that the piston is about to reach the top of its stroke. The timing is set this way to ensure the pressure has time to begin pushing the piston back down. When the spark plug is fired with the incorrect ignition timing or fuel mixture the pressure waves move out from the flame wall faster than the explosion itself. When this happens they impact the side wall at the incorrect time and a sound wave is created and detected. The element in the knock sensor is designed to detect this sound and alert the computer to adjust the timing.

All sensors in a vehicle are extremely important and are generally overlooked on basic maintenance. During automotive check ups it is important to remember to check the smaller parts such as sensors and filters to ensure protection from future issues.