Check engine light is on with the key off and out!

[4byT100]

I just put a JDM 5VZ in my '95 T100. It was a big job for me! I finished the install today and it fired right up and I thought everything was wonderful but after running about 20 min. of driving around the block checking it out it stalled and wouldn't start again. When I turned of the key and pulled it out the check engine light was on and it stayed on even after I locked the door and walked away from it. It seem to be running great, all my hard work had paid off. I put a code reader on it but it couldn't connect. I'm a machinist not a mechanic and I don't know where to go from here. Any idea what would cause this to happen? I'll take it to a shop if I have to but I wanted to own this job. I'm so close! Thanks

 

[SR587]

if you swapped the computer maybe the reading from the key is effecting it.