Are you sure that it's overheating, or is it just a rise in the temp gauge or warning light?
Any dramatic signs like boiling (steam), or bad performance? Overheating could be caused by blocked cooling system (easy fix), maladjusted engine timing, poor fuel, overloading or high speed in hot weather. where do you live?
Hi, all these suggestions are worth while and you can add to the list water pressure test, check thermostat, radiator blockage & the water pump impeller etc.
But what you have not told us is what are the indications of overheating,
1 high temp gauge
2 lose of water
3 the water boils
If the dealer cannot get one of this events to happen, it becomes a problem to repair the car, in cases like this I would let the dealer drive it for a few days [workshop foreman etc] to see if he can get evidence that the car has an overheating problem. Also realize that the dealer gets paid for warranty work from Kia .
Regards Peter [auto machinist/engine rebuilder-motor mechanic-dip of electronics]