Oldsmobile/Buick Repair: Service Engine Soon Light, cutlass ciera, olsmobile cutlass

Question
Hello.I own a 1991 Olsmobile Cutlass Ciera SL,6 cyl. 3.3 L.I've had a major tune up done on the car and it runs great.I have noticed the service engine soon light comes on when I go faster than 40 MPH but goes off when I slow down but comes on again when I speed up.In addition to the tune up I had a ECM and oxygen sensor installed.The car has 166,700 mi. on it but runs good.It has no obvious problems, so why does the light come on and off?Thank you for your time and attention.

Answer
hi elizabeth.well gm spent 200 thousand dollars designing a way to make this light come on,lol since they have spent millions trying to get it to shut off.if there are no codes then most likely a speed sensor.which is basicly an error in the sensor that will most like ly never do anything but annoy you with the light.good luck please take a sec to rate my help ty billymac