Check engine light came on the other day on my 1997 Intrepid 3.3 I did the key on, key off method and got a code 21 - something related to the O2 sensor. So I take the car to Advanced Auto to have them run a scan on it to get a better idea of which O2 sensor. They gave me a printout that said: Hex Code 20 - No Definition Found. What's up with that? They told me take it to the dealer. Fat chance!