Temperature sensors have an extremely low failure rate because there's just one small part inside them. Based on your observation of the radiator fan turning on, it is a far better suspect the terminals in the connector are spread and not making good contact. That is the same as unplugging it, and that will default to turning the fan on as a precaution in case the engine gets too hot. The Engine Computer can't know temperature when the coolant temperature sensor circuit isn't working, so it turns the fan on to be safe.
There's a few different ways to verify this. The first is when the fan turns on, you should see the Check Engine light turn on too. There will be a diagnostic fault code that can be read by the people at most auto parts stores. They'll do that for you for free. For this symptom, expect to see a fault code related to "Coolant temperature sensor voltage too high".
Another thing you can do is when the fan is running when it shouldn't be, pop the hood and wiggle the connector very slowly. Once the terminals make a momentary good connection, it can take a few seconds for the computer to turn the fan off.
The third way to see this problem is with a scanner. Select the "Sensor data" or similar list from the drop-down menu, then look at the "CTS Volts" That's "coolant temperature sensor voltage". When the circuit is working properly, that voltage will remain between approximately 0.5 volts and 4.5 volts. A bad connection, or unplugging the sensor will send that voltage to 5.0 volts. That is what triggers the fault code and is what tells the computer to turn the radiator fan on.
First check the connector terminals for signs of corrosion. If you see that, replace the connector. You can order a new one from any auto parts store, or you can snip one off a similar car in the salvage yard. If the terminals look clean and shiny, they are likely spread too large to make a good contact with their mating terminals. Use a small pick to go in alongside each one and bend them a little to shrink their diameter. With many connector designs, you have to pull out the colored locking wedge to allow you to reach in and lift a plastic finger with the pick. That allows you to pull the terminal out by its wire, then you can squeeze them a little tighter with a small pliers. With this type of sensor, if you mix up the wires when you reinstall them, the circuit will work just fine. This sensor doesn't have a polarity.
This same problem can be caused by a cut wire for the sensor, but that is a lot less common. Spread or corroded terminals cause most of the trouble.
Tuesday, January 14th, 2020 AT 1:51 PM