2009 Audi A4 brake light bulb lamp replacement procedure: Purchase the correct replacement brake light bulb(s) first, before disassembling the vehicle. See sources and related links below for replacement brake light bulb information. Then consult your owner's manual for the replacement procedure. As simple as it sounds, the owner's manual is the best place to start for brake light bulb lamp replacement instructions and illustrations. You may want to consider replacing both left and right at the same time in order to keep brightness and color equal (optional).
Replace the brake pads.
Pull the carpeting behind the brake light. Unclip the assembly, replace bulb.
As of 2014, the cost for a Audi A4 brake light is between 12 and 15 dollars. This light can be purchased at stores like Auto Zone.
This site has step by step instructions (with photos), perhaps the A4 is similar. http://audidiy.blogspot.com/2010/04/how-to-replace-brake-light-bulb-on-audi.html
The brake light in the Audi A4 display means there is an issue with the brake system. The most common cause of this warning light is low brake fluid.
You need to replace the break pads or you can bodge it up by connecting the two wires
By going to the nearest service!
you go.... and replace it!
Faulty ABS unit .
replace brake line
To change the brake light bulb in a 2002 Audi A6, first, open the trunk and remove the access panel on the rear light assembly. Disconnect the electrical connector from the bulb holder and twist the holder counterclockwise to remove it. Replace the old bulb with a new one, ensuring it's securely in place, then reassemble the components by reversing the steps. Finally, test the brake light to ensure it functions correctly.
A bad brake sensor can cause the ESP light to stay on in an Audi. Other reasons the light stays on include a problem with the batter, loose clamp causing a leak in the intake, and worn brake pads.