What causes the ABS warning light to stay on in an Audi A4?

I recently noticed that the ABS warning light on my Audi A4 stays on even after I start my car. What could be the cause, and how can I fix it?

The most common cause for the ABS warning light to stay on in an Audi A4 is a faulty wheel speed sensor. Check all the sensors and replace any that are damaged or malfunctioning.

Another possible cause for the ABS warning light to stay on in your Audi A4 is a faulty ABS module. This component is responsible for relaying information from the sensors to the control unit. Replace it if necessary.

It could also be a malfunction in the brake system itself, such as air in the brake lines or a faulty master cylinder. Have a mechanic check your brakes to determine the cause of the ABS warning light.