Yup it's the thermostat. The OEM one has a tendency to stick open. Since it can't close the coolant continues to cool when the car doesn't need it to and you say hello to Mr. CEL.
I actually just replaced my thermostat for the same problem a couple of days ago and it fixed the problem immediately (well after I reset the code). It came on at the very end of a 5-7 hour highway drive, but even after that the car ran fine and didn't appear to have an issue heating up. After replacing it the car clearly heats up more quickly.
If you have some standard DIY skills you can replace it. Getting to it's a little tight but is much easier from the top of the engine (plus then antifreeze doesn't spill on you). I recommend removing the tensionor pulley assembly from the engine block as the thermostat is right underneath it. You'll need:
-various socket extensions (it's tight getting the bolts holding the thermostat on)
-large pliers (for compressing hose clamps)
-container for coolant (plus funnel, etc.)
-ramps or stands/jack