One problem related to brake antilock control unit/module has been reported for the 2002 Audi Quattro. The most recently reported issues are listed below.
I have intermittent problems where the abs and brake lights go on with an accompanying three beeps from the console of my 2000 Audi a6 Quattro sedan. This is happening most of the time now. The computer diagnostic indicates that there is no communications with the abs system. My mechanic tells me that he can fix this for about 25% of the value of my car. This is nuts. I go online and find that there are dozens of people with the same problem that have filed questions with various Audi-related web pages, all with the same or nearly the same problem. This appears to be a design flaw not a worn-out part. I along with many others are terribly frustrated with this issue.