Hello my name is ***** ***** thank you for your question.
If you have a P0505 code, you need to remove and replace the IAC and Ecm both at the same time to resolve your issue. Then if you have any other codes, then you would then diagnose the other codes.
The reason why you don't worry about any other codes is cause the IAC ( idle air control ) damages the Ecm. The IAC has coolant going around the chamber to cool it. So there is a gasket for the IAC. ( Idle Air Control ). When this gasket fails, coolant seeps onto the IAC motors and shorts them out. When this happens your Idle will raise, be irratic, the car won't start and do all sorts of idle type issues, then it will get into running issues. But also, at this point when the IAC gets shorted, it also damages the ECM where it would control the IAC. So if you were to replace the IAC, the Ecm since that circuit is damaged, will damage the new IAC. So what needs to be done to correct this, is to replace the IAC and the ECM both at the same time. Other wise you will never get a head of the issue. Then you will need to reprogram all your keys for your car and have a Idle relearn performed.
I made a video on this. It's 6 parts cause back when I made them I could not upload a long enough video. See link.
If you have any more questions on this, please feel free to ask.
If not, and you're satisfied with my help, please Rate me. That is how I get credit for my work. I hope I have
provided you with Excellent Service.