If your vehicle is showing engine idles too high, here are the most likely causes and the parts you'll need to fix each one.
Unmetered air entering the engine causes the computer to add fuel, raising the idle speed. This is incredibly common after recent engine work where a hose was accidentally left disconnected.
The IAC valve regulates the air bypassing the throttle plate to maintain a steady idle. Carbon buildup can cause it to stick open, allowing too much air into the engine.
Carbon and sludge buildup inside the throttle body can prevent the butterfly valve from closing completely when you take your foot off the gas pedal.
If this sensor fails and constantly tells the engine computer that the coolant is freezing cold, the computer will permanently keep the engine in its high-RPM 'warm up' mode.
The PCV valve routes crankcase gases back into the intake. If it gets stuck in the fully open position, it acts like a massive vacuum leak, driving up the idle.
The gasket sealing the intake manifold to the engine block can degrade over time or be installed improperly, allowing a steady stream of unmetered air into the cylinders.