Knock Sensor 1 - G61-, Checking: General Description
WARNING: This page is about a different car, the 2018 Audi A3 Quattro, 2017 Audi A3 Quattro, 2016 Audi A3 Quattro, 2016 Audi A3, and 2015 Audi A3 Quattro. However, it is still accessible from the selected car via links, so may be relevant.
The Knock Sensor 1 -G61- is a tuned accelerometer on the engine which converts engine vibration to an electrical signal. The Engine Control Module -J623- uses this signal to determine the presence of engine knock and to retard spark timing, if necessary.