Here's a stupid question, but I have to ask... Are you plugging into the ECM test connector, or possibly the ABS test port by mistake? I don't remember off the top of my head if the SC has both test ports up by the passenger side firewall, but they are on my SHO. They look identical with the exception of the second lead with the little square end connector on the ECM test port.
Basically, unless the test port harness is bad, the ECM will run through a series of system tests just before it generates the codes. You should hear the lowspeed and highspeed fans, the fuel pump, and a bunch of relays cycle on and off. Due to the fact that the car will crank, it sounds like you either have a bad test port connector from the ECM, a bad code reader, or you are plugged into the wrong port (ABS).