99 Dodge Durango idle problems

I have a 99 dodge durango. It has a sound 5.9 motor in it and runs fine. Until I turn on the A/C. So the check engine light kept turning on and off, I bought a code reader and it gave me a code PO505 and said Idle control system malfinction. Can anyone give me a clue on what that may mean. I would apperciate you input!

You can save some time by just replacing the ISC valve, or if you want to try cleaning it remove the throttle body and remove the valve from the throttle body then clean the carbon deposits out of the valve and clean the throttle body inside also, reassemble everything and clear the code.