I had been getting a P0305 code (misfire cyl. 5), so I decided to try the easy fixes first, which included replacement of the #5 injector. The P0305 code went away, but now I have a P0505 code (idle air control issue). Is it possible that I didn't seat the injectors properly? Would that be a possible cause? Or would it be more of a vacuum hose issue?

I noticed that the code appeared before I started the motor. That being the case I guess I'm leaning toward a sensor or electrical connection. Any suggestions where to turn next?

 

Your problem is going to be with the IAC motor. First thing I would check is that it is plugged in properly, and the harness didn't get put somewhere that it could be damaged by anything. It's on the back side of the throttle body, so, usually it's pretty safe. It may be as simple as the throttle body needing a good cleaning, or, it may be a failed PCM. (which would be just a coincidence, unrelated to other repairs.)