HKS,
The same thing happened to me 5 years ago. I forget who it was on the other forum but they told me it was a manual, confirmed via PM and sent me an Auto unit anyway.
At the time, I could plug it into Nissan Datascan thing and it would tell me it threw a trans code. (Since there was no auto there, only manual, I expected it)
I have since acquired a manual G20 ECU that is truly manual from TopDog on here and the light went away and has never returned. I highly doubt the ECU is throwing a code unless you gave it a reason to. Either something changed or pre-existing condition caused it.