Hello, and thank you for choosing Just Answer!
There is a common problem when fuel injected engines idle too high, though it could be something as simple as a vacuum leak below the throttle plate, it is usually a little mor complicated. See when fuel injected engines are idling, the throttle is completely closed, and the air entering the engine comes through a bypass that has a valve on it (which he computer controlls). This is how the computer measures the amount of air entering the engine while it is idling. The valve opens and closes as demand is needed, like if you turn on the AC, the engine needs a little more power...things like that.
The piece that controlls this is the IAC. But this will not be something as simple as changing the IAC...because it is computer controlled, and if there is a short in the IAC motor(which moves the valve), then the circuit in the computer that controls the motor would also be damaged.
With that said, you can try changing the IAC and if that doesn't fix it, you would have to change the computer, too. But you would be FAR better off, getting it to a shop, so a technician could put it on a scantool, and everything could be monitored in real time, while the engine is running. The fuel injected engine's idle system is actually very complicated and actually needs to be hooked to a scantool for proper diagnosis.
But YES, it soulds like the IAC is stuck wide open, as long as you have verified that there are no vacuum leaks in the engine. A quick way to tell off on vacuul leaks is by using a can of carb cleaner and spray it along all the hoses, fittings, and gasket surfaces for the intake part of the engine, and when the idle speed shanges, you have found the leak. Another good place to look is for internal vacuum leak, which would be a PCV valve leaking...so while you're at it, change it anyways. As it is an often neglected piece of the engine that requires regular maintenance. If it is leaking the engine will idle higher than normal, and even use some oil.