service engine soon

I have a 94 Buick Skylark with an occasional SES light coming on, primarily when going down a long hill or when slowing down to a stop from highway speeds. I tried to read the code from the dashboard by jumpering the pins on the diagnostic connect per Chiltons, but I cant seem to get it to work. I have read the connector on 94 and 95 GM cars may have transitioned to an OBD1.5 format. I have replaced the O2 sensor on the exhaust manifold and checked the MAP sensor. Any other ideas?

John Lumia
1994 Buick Skylark
Louisville, CO

I forget the year that it started, but that terminal jumping ended (I thought) with OBD1. I had a 91 Beretta that wouldn't do it, and my mom had a 94 blazer that wouldn't do it.

AutoZone or PepBoys will read the code with a scanner for free.