1999 Chevy Astro key fob won't program

I just got a new fob for my van. I had it tested (using a GM fob tester) and saw the lights light up(on the tester) when the buttons were pushed on my fob. I then followed the instructions to short pin #4 to pin #8 on my OBD II aldl connector (top row, 4th from left and very right). The locks cycled as expected and then I pressed the fob buttons (lock & unlock together) for more than the requested 10 seconds, but nothing happened. I repeated the attempt with the ignition on and off with no success. My Snap-On Modis doesn't help me here either. Suggestions?