Upon starting, engine idles at about 1200 in park. When it warms up, idle drops to nothing, unless air conditioner is at max where rpm is roughly 400 to 500. Where should I start looking?
Start by cleaning the throttle body. The air filter removes 99.9% or air particles. That 0.1% build up on the throttle plates over time. That buildup restricts the minimum air rate.
When a car is idling not all the air passes through the IAC (idle air control valve). Some of the air passes through the throttle plate. The throttle plate is held slightly open by the throttle stop or there is a small hole in the throttle plate. (what ever you do DON'T mess with the throttle stop) When the car starts cold the IAC opens all the way to make sure the car starts. At this point the minimum air rate is not important because the IAC is drawing in so much air. As the car warms up the IAC backs off lowering the idle. At this point the minimum air rate becomes more and more important because with the lower idle the minimum air rate can account for 50% or more of the incoming air. While driving, when the throttle closes the car can't adjust the idle fast enough and the engine stalls.
With the AC on the car automatically bums up the idle. This opens the IAC possible far enough to keep the car idling. I'm almost willing to bet that if you start the car cold with the AC off. Then allows it to fully warm up without touching the throttle it will continue to idle.
Clean the throttle with throttle body cleaner or carburetor cleaner and a tooth brush. Be sure to clean the throttle bore and both sides of throttle plate. If there are any holes in the plate make sure to clean then too. Finally, immediately after cleaning the pates start the car and allow it to idle. Treat the car as though it was flooded, push the throttle down as your starting it. The cleaner fumes tend to build up while cleaning and may flood the engine.
All modern vehicles require a scan when it comes to fault finding. Even as a routine part of its service a scan should be included. Your vehicles idling is controlled by APP- accelerator pedal position switch, IAC- idle air control and the ECT- engine temperature sensor. The engines RPM is also increased by a pressure sensor in your AirCon system. The engine cooling temperature sensor is usually a dual circuit affair. When the engine is on the cold side the primary circuit of this sensor indicates the temperature to the engine computer, and the engine computer adjusts the engine speed accordingly. As the engine warms to operating temperature, the secondry circuit takes over and controls the idle speed. The dual circuits provide a more precise temperature reading over the engines heat range. When you load the engine with the AirCon a further increase in speed is provided to counteract the drop in engine RPM from the AirCon loading. The most probable cause of your problem is the ETC, but a scan should be carried out to truly determine the problem area.