Sounds like the coolant temp sensor it's screwed into ur head below where ur upper radiator hose goes onto the head. Unplug it by squeezing it look at the connector see if it's corroded If it is u will have to clean or replace the connector. If u take the sensor out u can see if the plastic tip of it is cracked they crack sometimes when they go bad. Just becareful u lose coolant when u take the sensor out make sure it's cold. The sensor is under 20 if u replace it make sure u get a brass tip one. Let me know what u find.