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If your vehicle was built prior to 1996, you will need an OBD I scanner.  Vehicles made after 1996 will need an OBD II scanner.  Connect the code scanner to the port beneath the dashboard and turn it on.  Once connected, use the scanner to bring up the error codes making the check engine light come on.  In some code scanners, it will provide the English description of the error code on the read out. If your code scanner does not, you will need to look up the code the scanner shows you to find out what it is. Insert the key into the ignition and start the engine.  Wait a minute for the engine idle to level off, then use a pen and paper to write down the RPMs that your engine settles and idles at.  You may need to let the engine run for a few minutes in order for it to come down to its normal idle speed. Keep the paper you write the RPMs on handy so you can compare it to the engine’s idle as you continue the test. With the idle RPMs noted, shut the vehicle back off and remove the key from the ignition.  Open the hood and locate the Idle Control Motor.  Once you locate it, disconnect it.  The way to disconnect your Idle Control Motor will vary from application to application; refer to your vehicle’s service manual for more specific instruction as to how to disconnect it. You can usually disconnect the idle control motor by unplugging the wiring clip going into it. With the Idle Control Motor disconnected, get back into the vehicle and start the engine again.  The engine should still start and run without any issue despite the idle control motor being disconnected. Be sure to tuck the wiring you disconnected up in the engine bay so it doesn’t get caught in any moving parts. Allow the engine to run for a minute so it settles to a consistent idle, then take note of any difference in the idle RPMs versus the idle you observed earlier.  If the idle has not changed at all, it’s likely because your idle control motor has not been functioning.  If the idle control motor was working, disconnecting it should have created a difference in the engine’s idle. This test confirms there’s an issue with the idle control motor, but not what the issue may be. If you identified a difference in engine idle RPMs with the Idle control motor connected and disconnected, confirm your hypothesis by reconnecting it and starting the engine again.  The idle should go back to what it was when the idle control motor was last connected. Refer to the note you took about idle RPMs to compare it to your first reading.

Summary:
Use a code scanner to check error codes. Start the engine and take note of the idle RPMs. Shut the car off and disconnect the Idle Control Motor. Restart the engine. Look for a difference in idle speed. Shut off the engine and reconnect the Idle Control Motor.