Here's also another method of displaying the codes on the speedo.
http://www.corvettephotographs.com/c4vettes/codes.htm or if you want the codes, and record live data.
http://www.ttspowersystems.com/DataMaster.html You would need an ALDL cable though, and a pc with a serial connector. I think the usb's might work, but harder to get it to communicate on some pc's. Software is free for the first 20 recordings.