2003 Honda Accord Question: Transmission or other?? code P0506 and P420 come up....

My son bought a 2003 Honda Accord automatic, 1 week after he bought it, it started to idle really high before it would go into 2 or 3rd gear (sorry not sure which one)... I started looking into this and found it was a common problem. I called Honda to see abt a recall and was told since i wasnt the original owner, there is nothing they will do to help... WOW!!! So you only stand behind your cars if there is only one owner?? Very Disappointing. However, he drove it like that for a little while, then it just stopped and didnt do it for abt 2 mnths. Well I started driving it and it started doing it again...it will idle really high and then eventually go into gear, I can always shift it into 2nd and it will go in better... but still.... so he took it to Honda dealership who said it was the transmission, $4g to fix.... took it somewhere else and $2800 to rebuild.... NOW, took it to another mechanic, just to make sure, bc that is ALOT of money...and he said the car is throwing 2 codes... he drove it and doesnt seem to think it is the transmission...since it doesnt do it all the time.... one code is the P0506, idle control.... which he believes is throwing the other code, P420 for the catalytic converter... he feels that if he gets the throttle body taken care of, it will fix the other issues.... does this sound right? Could it be something causing it to idle really high sometimes when it goes into 2nd or 3rd gear and not be a complete transmission failure