Remove the air cleaner hose. Start engine and cover the ports inside the throttle body. If the upper one reduces the idling speed, the IAC is the problem.
If covering the lower port lowers the idling, the cold start (Fast Idle Thermal Valve) is faulty. Check the coolant hoses for clogging. The FITV plastic valve could have worked itself loose due to a bad/hardened O ring allowing excessive air into the manifold and increasing the idling speed.