A word of advice: there is a TSB floating around that covers various Hyundai/Kia vehicles regarding a P0128 code. Apparently, the ECU's cooling system logic gets easily confused if the system has anything more than a 50/50 antifreeze mix in the system. Past 50% straight coolant, and the computer can get confused.
I have personally run into this after doing a coolant flush on said vehicles. If the ratio isn't spot on, a code you shall see.
Does the TSB provide a fix? Or is only using exactly 50-50 mix their "fix"?