The IAC (Idle Air Control) valve on a 2005 Chevy Colorado is located on the
throttle body. It's a small, usually cylindrical valve mounted directly to the throttle body itself. You'll need to look closely at the throttle body; it's often near the top or side, and may have a single electrical connector attached.
To be certain, you might want to consult a repair manual specific to your 2005 Chevy Colorado's engine (2.8L, 3.5L, or 3.7L) for a diagram and precise location. A picture search online for "[your engine size] 2005 Chevy Colorado throttle body" will also likely show you where it is.
