The number one cause for high idle is an imbalance in air pressure. An engine that is idling too high may be caused by a faulty idle air control motor valve that has completely failed or that has simply become weak and is not functioning correctly.
Another cause of high idle could be the air intake boot which delivers air into the intake system. If it's broken or leaking, it can cause an imbalance and create high idle. The throttle must also be functioning correctly and be free of build-up. Inspecting the intake gaskets and engine vacuum hoses can often lead to the problem. If something is wrong with those, the appropriate course of action is to clean the lines or replace them.
High idle can also be caused by something as simple as damaged or faulty spark plugs, and the solution would be to replace them. Spark plugs that are improperly installed can also cause high or surging idle. A dirty or damaged fuel injector could also be the cause. Sometimes, an inexpensive fuel cleaner can clean clogged fuel lines. The problem could also be the ignition wires or the carburettor. If the diagnostic light is on, vehicle parts stores, such as Auto Zone, can do a diagnostic test for free to identify the issue.