Ok, My kia, I replaced the thermostat 2weekends ago, now today on my way home the check engine light came on, but the engine didn't overheat. The needle didnt go about half way. So I swang into advanced auto and got a Thermostat code. I thinkit was p0107 or close to that. The guy told me the thermostat was more then likely stuck open. I just replaced one that stuck closed, now I get one that is stuck open. What is going on?
I am at wits end with this kia, and can't afford 67.00 bucks to replace cluth fan and I do all my own work on this kia, that I possibly can. Any other ideas as to what I am doing wrong? How do i test to see if the thermo is good? The guy mentioned boil it? What does that mean? What do i do?

Thanks for your help. I need the vehicle to get back and forth to work which is 47 miles one way.