My car idle idling high in drive and revving to over 2000 RPM when its put in park

Will replacing my idle air control valve fix this? My 2000 Mitsubishi Galant (2.4L I4) is idling high while in drive or reverse (around 1100 rpm), and when it is put in neutral or park it revs to over 2000 rpm very quickly. The car runs fine otherwise, but my gas mileage is suffering (by about 25%).

From everything I can find online it appears to be my idle air control valve, but when I brought this up to a mechanic during some other repairs he seemed to dismiss it. I also removed the IAC to make sure it wasn't burned out; it looks fine and moves in and out when the car is started and different gears are selected.

The IAC valve malfunction is one of 3 check engine codes in my computer right now(P0505). The others are P0170 and P0400. It's an expensive part so I want to make sure that it is causing the problem before I buy it.