Truck Repair: over heating, diagnostics test, air cooler

Question
My truck keeps over heating when I turn the AC on or let it sit in Idle,did a diagnostics test on it and was fine.so my qeustion is,is it the thermostat.

Answer
what type of engine?could be thermostat,water pump.Something you can try is remove the air charge cooler in front of the radiator.flush dirt out of air cooler and radiator with compressed air.if you have a compressor,you can do it yourself.it takes a good little bit of time to do correctly. if you dont remove the "air to air" you usually just blow the dirt back and forth between it and the radiator.thats about all i can do to help.any more questions just post.thank you.