I have an 1991 Olds Cutlas Cierra S with 138,000. It has the 3300 6 cyl. in it. About a week ago the service engine soon light came on. The car drives fine. No issues starting it, at speed, when cold warm etc. Runs like a champ. A few local mechanics said that code 26 ment that the Quad Modual Driver was bad and the ECM needed to be replaced. This I have done. Drom it for a while and all was fine. After about 70 miles the SES light came on again. Checked for codes and sure enough it was code 26. I have searched the internet with little luck finding any new information. Here in NY if the SES light is on the car fail inspection so I have to get it fixed. Please let me know if you have any ideas.