The knock sensor is an important part of the car's ignition system. It functions by detecting instances of detonation or "knocking" within the engine and sending a signal to the vehicle's computer that in turn adjusts the ignition timing and fuel injection. The knock sensor is essentially a microphone that "listens" for certain vibrations.
So what is detonation? Detonation is when combustion of the air/fuel mixture in each cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion flame front. The air/fuel mixture is only meant to be ignited by the spark plug and also when the piston is at the right point in the stroke of the engine. Any ignition outside the normal cycle creates the pinging or knocking noise you hear. Over time, the knocking creates increased heat and stress on the pistons, eventually leading to engine damage.
It sounds a bit complicated, but the knock sensors keep all of that under control. They also give you the ability to run a lower octane fuel in a pinch at the cost of engine performance.
Typically, a knock sensor fault will trigger a check engine light and a fault code. Use a diagnostic software package or the VAG-COM computer to read the codes and determine which sensor is bad. Unfortunately, replacing the knock sensors requires removing the serpentine belt, the alternator and also the secondary air pump. See our articles on Serpentine Belt Replacement, Alternator Replacement and Secondary Air Pump Replacement for more information.
When installing a new sensor, take care to align and torque the fastening bolts precisely. Failure to do so could result in a sensor not working correctly. It is also important to align the sensor so that the wiring harness does not come in contact with either of the sensor housings. A stray brush across the sensor by the wire could be read as detonation. Once installed, torque the securing bolt to 20Nm (14.75 ft/lbs.).
Shown here are the two knock sensors located on the front of the engine block behind the alternator and the coil pack. Use a small screwdriver to press the tabs (green arrows) on both electrical connectors and pull them off.
Use a 13mm socket to loosen and remove the 13mm bolts (green arrows) holding both sensors to the engine block. When installing the new sensors, make sure you orient them in the same direction as the old ones. Torque the 13mm bolts to 20Nm (14.75 ft/lbs.).