Disconnect battery first. Remove the throttle body so you can clean out the IAC port properly.

 

What purpose on disconnecting battery? Just to reset the PCM or to protect the IAC. I'm having a similar series of events. I did a diagnostic on mine, which involved turning the ign switch on, then I cleaned it with TB cleaner. Afterward, the CEL was on, but it went away after a couple driving cycles. I thought cleaning the IAC valve & the port did the trick (they were nasty), but the problem reared itself yesterday, but didn't today. I have to keep my foot on gas pedal at least until it warms up, then it's good. Should I just replace the IAC valve? could it be something electronically wrong with it? Also, I should mention that when I turn on the heater at anytime, I can hear a hissing from what sounds like to me might be a vacuum leak from inside the dash and when I turn the truck off, the hissing/high-pitched sound is still there and gradually fades away. Could this be related? I haven't had this truck long & this is first winter I've driven it, so some things I'm not sure about being normal or what.

 

You disconnect the battery otherwise the computer may not recognize the new part....so, even if you change a bad part with a new one it will still run the same.....until it cycles through....

 

Yeah, my check engine light came on after I cleaned the IAC valve and ran one simple diagnostic on it (I made sure the pintle would retract after turning key switch on). The CEL stayed on for about 2 or 3 driving cycles like I expected it would. But, as soon as the outside temperature dropped below 40degF again, the same problem appeared. From a cold start, it'll fire up but immediately quit. But, if I give it some throttle at start and give enough to keep the RPM around 1000 for about 30 seconds, it'll hold idle on its own after that.