Interpreting car codes- what do I need?

[Ti Kan]

Rick Liotta writes:
> I have a 99 A4 Quattro and its yellow 'check engine' light just came on at 
> about 75500 miles (maybe a little less miles).  The car is running fine.
> I would like to know how to interface with the car's brain.
> Please-- what do I need to access and interpret any codes that are car is 
> giving-- software? Interface cable?

VAG-COM: http://www.ross-tech.com
or VAD (formerly Prodiag): http://versatileautomotivediagnostics.com/

-Ti