Chevrolet Repair: Transmision codes, failure code, chevy g30

Question
I have a 1992 Motorhome on a chevy G30 1 ton van chasy. The motor is a 454 and the trans. is a 4L80E. The trans is in limp mode now. Two weeks ago it did the same thing and when I was going to Autozone to get the codes read it started working just fine and the check engine lite went off. Now the lite is back on and the transmision won't shift, When we hook it up to a scanner the OBD will not comunicate. 2 weeks ago after the light went off I jumped the plug with a key that I got from Autozone to see if there was any memory in the codes and it gave me a code 12. So it was working 2 weeks ago. Any addvise? I have get the codes before I can have the transmision fixed.

Answer
The CODE 12 is ONLY a test code that is built into the computer that tells the technician that the system is able to recognize and store any other fault codes should something faile. This code 12 is NOT any type of failure code.

Yes, you will need to get a code(s) from the computer before a diagnosis can be made on the trans.