Coolant temperature sensors are located on or near the thermostat housing. They are used by the computer for several things. They are used to sense the temperature of the engine at the computer to adjust the fuel mixture. When the computer senses that the engine is cold through the use of the sensor it will increase the on-time of the injector. This enriches the fuel mixture the same as using the choke. It also uses the sensor to determine when to turn the engine cooling fans on and off.
Allow the engine to cool to the touch. Place the dripping pan under the radiator petcock. Open and let about 1 gallon of antifreeze to drain before closing. Disconnect the electrical connector on the coolant temperature sensor.
Remove the sensor with the appropriate wrench (the size will depend on the year, make and model of your car). Install Teflon tape around the threads of the new coolant temperature sensor.
Install the coolant temperature sensor and tighten just enough to feel that it is tight. Do not torque down on it, or it will distort and fail to work, or give false readings. Reconnect the electrical connector on the sensor.
Refill the radiator with antifreeze. Leave the radiator cap off. Start the vehicle and let it warm up. Watch for the thermostat to open. If you look in the radiator as it reaches 90.6 degrees C, the thermostat will open, and the fluid in the radiator will drop as the antifreeze begins to circulate and expels any trapped air. At this point, fill the radiator the rest of the way, and install the cap. Check to make sure the overflow bottle is half full.
Keep your eye on the temperature and the electric engine fans to see when they come on. They should come on when the temperature gauge shows just over half or at about 101 degrees C. If they work properly, you are finished. If they fail to come on, then there is an electrical problem. The coolant temperature sensor is defective, or the fans are no good.