Update: Message popped up again one morning, and was soon followed by a check engine light with the code P0128 (thermostat). Since the thermostat seemed to be working fine (plenty of heat, reaching normal temp in a reasonable amount of time), I bet on the coolant temp sensor and replaced it. A week later, same problem occurs. Problem will either go away on its own by itself (message disappears, temp gauge starts reading normal), or a battery reset (removing negative cable) will clear it out. Works for a few days, then bam! happens again. Obviously something in the electrical/sensor system is triggering the failsafe mode, but I have no idea. Thermostat after all? Air in coolant? (I've read that might create a problem) Other than the aforementioned, everything seems to run fine. Any other ideas?