Hey y'all, my Accord (2000, F23a4 shortblock/F22a hybrid) has been having this issue where I turn it on, and the idle jumps to like 5-6k. I recently swapped out the old stock F23a4 for an F22a SOHC non-VTEC head. I had lost my OBD II scanner and so for the longest I had no idea whatsoever what was causing this issue. I had suspected it was either an intake vacuum leak, or a problem with the IACV. But I found my OBD scanner the other day, hopped in my car, plugged it in, and put the ignition in the on position.

I found the only code I had was a P1259 (which was totally to be expected, because I had swapped the cylinder head on my F23a4 with an F22a, which does not have VTEC.) There was none for high idle speed or unmetered air intake or anything like that.
So my question is, what could be causing my car to idle so high when I start it up? One of my mechanic friends thinks it could be the timing, but I don't think so since I made sure the timing was damn near perfect when I put the belt on, and I even made sure I got the correct belt for this swap (VTEC and non VTEC belts are not directly compatible.). I was told by someone on Reddit that it could be since I didn't change my ECU to match the cylinder head I have, that it could be causing the idle problem. So if I need a new ECU, which one should I get? I found a video on YouTube of a guy who basically did the same swap on his Accord as I did mine, and he used a P12 from the early 90's Prelude. I could do this but it would also require me to use an OBD II-I jumper harness. Another option I was thinking was the ECU out of the 96-97 Accord LX, which is also OBD II and came in non-VTEC trims. What could be causing my idle to **** up, and if changes in engine management are necessary, what route should I go for this?