Carbon Exhaust

New to this site so bear with me. My engine light came on (02 millenia), oh... about a week ago. I got a diagnostic done and they told me that it was the carbon exhaust. I didn't get any paperwork or codes, or whatever, they just suggested some place else I should go. What does that mean,... it's my carbon exhaust? Is my muffler not letting enough carbon out? Is my exhaust system bad? The car has been running fine, without any problems.