I have a 1991 Chevrolet Lumina 3.1L V6. The car runs real good, but when I start slowing down from highway speeds (60+ MPH) the SERVICE ENGINE SOON light comes on once the speed drops below 48 MPH. Once the light comes on it will stay on until you shut the engine off. The code that I got from the ALDL CONNECTOR is CODE 12 (Diagnostic). I have not been able to get any other codes. What could the problem be and how would I fix it?

have you noticed any performance/other issues due to light? Have no performance problems at all or other issues with the car right now except for the SERVICE ENGINE SOON LIGHT coming on. do you have to pass smog? Here in Texas, we have no smog test. There is no code 12 so you could be having a problem with the computer itself or you may be reading it incorrectly. I have a Haynes repair manual for my 1991 Chevy Lumina and in the manual it does show a CODE 12 (1 flash, pause, 2 flashes). This is what I am getting. Alldata says differently.
Code 12 just means that the ECM is going into diagnostic mode, this is NOT a trouble code! Code 12 should blink a couple of times pause then blink your trouble codes if you have any. If you have no trouble codes, it will resume to blink code 12 again.

I cann't get any other code than CODE 12. What should be my next step from here? Thank You. You're dealing with an issue that isn't setting a code yet putting the CEL on. I would be suspecting the computer itself. Is there some manual way that I can reset the computer? Who said anything about resetting? You can't reset anything but the block learn. My friend next door to me, told me that you could disconnect the car battery and let it sit overnight, the computer would shut down. Then in the morning, connect the battery back up and the computer would reset itself anew. If I took the car to a garage, could they check the computer to see if it is starting to go bad? Really? What does he do for living?