Search for the coolant temperature sensor; you will mount it on the front side of the cylinder head rear cover but it will screw in. For evaluating the characteristics of the sensor, the connector has to be disconnected, and the resistance between both terminals has to be checked, which will decrease with the increasing coolant temperature, allowing for the evaluation of the sensor's function. If the above monitoring is not effective, then the sensor should be changed. Remove the coolant or accept the fact that you are going to spill some of the liquid, then undo the screw on the sensor. To fix this new sensor, screw it in and make sure it is very tight, then cover the threads with teflon tape to avoid leakage.