Overheat telltale
 

Innovation/Volvo 600 with Mefi 3 ECM shows on Rinda Techmate that "engine overheat telltale has been set". Thinking Ok, dont know when this happened but...so cleared code. cycle power off/on (not running), and code is immediately back. Coolant temp sensor bad? Unplug it, clear code,cycle power, and code is immediately back again. Probably not coolant temp sensor? What else should I be looking for. When running, all engine functions are good and no audible alarm, such as would be if it was really overheating, or even thought it was?

Any thoughts or guidance is appreciated.