Oldsmobile/Buick Repair: SERV ENG SOON 1987, oldsmobile regency, smog test

Question
1987 Oldsmobile Regency Ninety-Eight model Brougham: "SERV ENG SOON" light comes on after engine warms up and won't go off. On Board computer yields code 13 Oxygen Sensor Circuit. Replaced Oxygen Sensor, disconnected battery ground to re-set computer, yet "SERV ENG SOON" light still comes on.

This problem is apparently causing CO readings to be high enough (1.15) to FAIL smog test 1.0 limit. Any thoughts?

Thanks,
Terry

Answer
HI TERRY..TUNE UP ,,O2 SENSOR CODE WILL SET ANYTIME THE FUEL IS NOT BURNING RIGHT.SAY ONE CYL IS BURNING FUEL FASTER OR SLOWER.THE EMISSIONS OUTPUT WILL BE DIFFERENT.THE O2 SENSOR WILL DETECT THIS AND SET FAULT CODE..GOOD LUCK PLEASE RATE MY SERVICE TY BILLYMAC