[fl0wmastr]

The thermostat is the #1 reason, but I also got this code when I didn't mix the coolant/water mixture right.
Too much water raises your freezing point and too much coolant lowers the flow rate of the fluid and triggers the code.
The slower fluid doesn't cool the engine as well. If you've already replaced the thermostat, I'd try flushing the coolant in the system.
When I flushed my system the code went away and never came back.

If that doesn't fix it, I'd replace the thermostat, coolant temp sensor, the water pump, and the expansion tank. Unless you know these have been replaced recently.