As far as I know (it was true in my 3g 'Lude), yes you should be able to read the trouble codes right off the ECU. There should be a blinking light visible when you pull the carpet away from it, but you shouldn't have to remove any metal parts. As I recall (someone correct me if I'm wrong), the light should blink once to show that it is working, and then it will show the code. A long blink is a "10" and a short blink is a "1." So, 2 long blinks and 3 short ones would be "23." Then consult a service manual (Chilton's, Hayne's, Helms) to see what the code means. If it is the O2 sensor, I think the code is just "1," but don't quote me on that.