Originally Posted by Inthemedia

Another thing to test. With engine at idle, turn on the AC to its MAX setting. Your idle speed should increase. If it does then the ICU is sensing this condition properly and at least this portion of the ICU is working. If the engine runs slower, then the ICU is not sensing the AC compressor is turning on. This indicates that the ICU is malfunctioning and needs attention. If the engine speed stays the same, then the ICU thinks that the A/C is ALWAYS ON and this could be the source of your high idle. Again this points to a problem with the ICU or even possibly the A/C Controller Unit (ACU) which generates this signal to the ICU.
T.C. Mann (aka TechMann)