Possibly the ECM itself. That'* the computer, right? My former H-Body was a 1989 Buick LeSabre and the SES light came once(I forget what code it was). The Buick(like my present Olds) did not come with any guages. A friend of mine had a Scan Tool and it showed the Alternator pumping out 19 Volts. He didn't think this was possible and surmised that it was probably the computer. Since you can't buy a computer and just return it if that does not solve the problem, we "borrowed" my mother-in-laws identicle 1989 LeSabre and swapped computers. Sure 'nuff, the same problem appeared in her car and mine was fine. I bought a new(rebuilt) computer and that was that. I'm sure there'* a better way to determine your prob, but this was my experience. Good Luck.
2006 Salsa Vibe
2002 Snap Orange Beetle - Turbo
1991 Olds "Orphaned" 88 - Donated to Science!