You can test your ECU but it would require you to adjust your manual idle control.

If you jump A & B or otherwise plug the IAC and adjust your manual screw to idle at about 600 rpm, the computer can't do anything about that.
If, when you hook the IAC back up or unblock it, it jumps to 1000 rpm again, you should see the IAC open farther and the computer would be commanding that. If it stays at 600 rpm, your throttle plates are open too far and your scan tool is lying to you.