If you know of someone with a VMS PC with a 6 pin connector you can borrow, try it on your cpach. It is designed for monitoring, but it has a lot of diagnostics similar to what a dealer would use, it will most likely tell you the reason for the warning.
I believe that the VMS PC is worth the price in what you will save in diagnostics alone.
http://www.silverleafelectronics.com/?q=node/6
Here is website, you can read about it and download a copy of the free software to take a look at. You can get the cable and black box for as low as $340.00 on the internet and with your own laptop or netbook, you are set. The new units come with a USB connector to hook it up to your laptop the current or older stock require a Serial to USB adaptor if your laptop does not have a Serial Port. If you like it and order, make sure you know whether you are getting it with the USB connector or need the adaptor.