Knock sensor

There is one knock sensor per bank. The ECU correlates the crank position with knock sensor activity, and thus gives each cylinder a "window" in time for which the knock voltage is valid. The two sensors are mounted roughly in the center of each bank, so they are closest to cylinders 2 and 5. This causes knock voltages to appear higher for those two cylinders. The knock sensors themselves are sensitive to a very narrow band of frequencies. The ECU does additional filtering before the knock voltages are used to determine ignition retard by the knock control system based on knock recognition.

Knock sensor activity can be monitored via VAG-COM in blocks 26 and 27. The resulting correction factors are in blocks 21 through 24.