The sensors are simply a wire embedded in the pad about 3 mm from the backing plate. When the pad is worn to 3 mm the wire wears through also and opens the circuit setting off the warning. It cannot be reset until you change the pads.

Thanks for the explanation! Do you know if the sensors need be changed along with the pads or can be reused? Although I have been changing brake pads and rotors for years, this is my first car with a wear sensor. All my previous cars had only the traditional spring indicator which just makes scraping noise when pad is too thin.

Thanks for the explanation, SchwarzS6.

The sensors are not reusable as they are built into the pads. When you throw out the pads the sensors go with it, You get new sensors with new pads.

OE pads comes with the sensor wire, Pretty straight forward.

https://cimg4.ibsrv.net/gimg/www.aud...a4a4bb0820.jpg
https://cimg8.ibsrv.net/gimg/www.aud...7d36d95c1d.jpg


This is the front brake wear sensor for B9 A4 on ECS Tuning. Trying to understand how it is attached to the pads. No sensor/wires come with the pad set though.

There is space on one of your front pads to slip the sensor into. Plenty of YouTube videos that show you how. Only on the drivers side front.

Thank you! Will look into it.

I am pretty sure these are built into the OEM pads. From what I can see the OEM catalog does not list a separate sensor wire. Are these for aftermarket pads?

It is OEM. You can find it in Audi parts site here: https://parts.audiusa.com/p/Audi_201...W0615121E.html
It appears there are two versions of pad set for B9 A4. There is also some confusion as to if the sensor is included or not with different version of the pad set.