ABS and Brake Light are on. on 2003 Chevrolet Trailblazer

I have a 2003 Chevy Trailblazer and my ABS light and Brake light came on after my dad replaced my breaks several months ago. My brakes work fine, and when I set the parking brake, the Brake light goes off, but turns back on when I release it. I have seen lots of postings for this for Trailblazers, but not really an answer about whether or not it is a big (ugh, expensive) problem or a cut wire, sensor problem, etc. I had my tired repaired (slow leek) 2 months ago and had them look at the cause of the problem, the mechanic said the brakes looked great and he didn't see any problem with ABS, but to be sure I would have to take it to the dealer, because he wouldn't work on the "brain box" (whatever its called)due to a lack of knowledge about it. Any help?

Asked by for the 2003 Chevrolet Trailblazer
Any quality shop with the proper test equipment should be able to check the ABS system on your 2003 Chevrolet Trailblazer for fault code(s). There is no way to determine the severity of the problem without first reading and diagnosing the fault code(s) stored in the ABS control module.