If your vehicle is showing engine idles unusually high when shifted into park or neutral, here are the most likely causes and the parts you'll need to fix each one.
Unmetered air is entering the engine through a cracked hose or failing gasket. The engine computer compensates by adding fuel, which raises the idle speed noticeably when the transmission load is removed in Park or Neutral.
A vacuum line or PCV hose was accidentally knocked loose or left disconnected during a recent oil change, air filter replacement, or engine repair, creating a massive vacuum leak.
The valve responsible for managing bypass air at idle gets clogged with carbon buildup and sticks open. This allows too much air into the intake, causing a high idle when the engine is not under load.
Carbon and sludge buildup inside the throttle body can prevent the butterfly valve from closing completely. This allows excess air to bypass the throttle plates, keeping the idle artificially high.
The sensor incorrectly tells the engine computer that the gas pedal is slightly pressed even when your foot is off it. The computer responds by commanding a higher idle speed.
If this sensor fails and constantly reads extremely cold, the engine computer will stay in 'warm-up' mode. This keeps the fast-idle engaged even after the engine is fully warmed up.