Ii indicates that the thermostat monitor has not achieved the required engine operating temperature within a specified amount of time after starting the engine.
If cooling system is not leaking and the fluid level is OK, replacing the thermostat usually takes care of the problems.
i´ve had that error code a few times myself. did´nt do anything about it.that was last year, never came back since. if it doesn´t stay on all the time, ignore it for a while