GM-GMC: 2003 GMC Sonoma Check Engine Light 86k, gmc sonoma, fuel cap

Question
While driving down the street my check engine light came on
at just over 86000 km. The vehicle is running fine. Any
idea what might be causing this?

Answer
There are literally thousands of possiblilities.  However, some are more common than others.  Check to ensure your fuel cap is tightened completely, if it is not, it can result in the MIL coming on.  The best way for you to find out would be to find somewhere with a scanner.  Many of the larger parts stores now offer 'code' reading as a free service.  Ask around and if you can get someone to read the code, write it down and I can help you further with the code number.
Hope this helps