1998 Taco engine rebuild, OBD2 issues

My son and I are rebuilding my 1998 Tacoma. After we rebuilt the engine it would not start. I plugged in an Innova 3100 OBD2 scanner, no powertrain codes present. So then I tried to start it and it started right up. Runs pretty good. I told my son to drive it around to see if any codes would appear, no codes appeared. Today we tried to start it and it would not start. So I plugged in the scanner and it started right up, but when I unplug the scanner it dies. I disconnect the cord from scanner, still runs, but as soon as I unplug the cord from the OBD2 port it dies. Any ideas?

 

I recommend double checking your ground connections.

 

Are you talking about the two grounds off of the battery?
We are going to try a used ECM.

 

Dorman68, you were right my son's friend found a loose ground wire, not sure where. But the issue is gone.
Thx for your suggestion.