I took my 2001 a4 into a smog check station yesterday and was told that it failed for two reasons: (1) the technician couldn't communicate with the car because of aftermarket stereo work that was done, and (2) the check engine light was on.

Any idea how I can unblock the communications and also, do I have to take my car in to a shop and resolve the check engine light situation before I can even pass a smog check?

Thanks in advance