Recently I had to replace the transmission in my 2002 Chevy Impala; it has 170,000 miles on it, but I wanted to keep it running for at least another couple years. Within 2 days of the having the new transmission..... which I had to sell a kidney to pay for, the Check Engine light came on...... Frustrating!
It was a fun project. The Nios softcore CPU was way overkill, but it was fun playing with the latest and greatest techology circa 2002.
I used an Elm323 dev. board to communicate to the OBDII port on my car using RS232 AT like commands. They still make a very similar board; Sparkfun also sells one too.
Link to Thesis
Link to PowerPoint