The P0505 just says that the IAC motor was not able to correct the idle to within 200 RPM of set point value and the IAC motor was pegged for greater than 5 seconds.
Anything that would cause a high idle (vacuum leak) can cause a P0505. I know on my 2009, if I place my ear next to the air cleaner and boot the ignition for 10 seconds (with kill switch ON, engine OFF), when I turn the key to off, I can hear the IAC motor step and "Park" itself.
Do you hear that happen?