You may have 2 separate issues here. Has this vehicle ever overheated? Heres what to check first. You need to disconnect the coolant temp switch and see if fans come on. With this switch disconnected, the fans should run all the time with the engine running. This switch is located on the bottom of the radiator. Click on the link below to see a diagram.
If the fans run with this switch disconnected, replace the switch.
Next you need to look at the sensor for the computer. This is the one which will give you the P0115 code. Click on this link to see its location.
This sensor is most easily tested with a scan tool using streaming data and a non contact thermometer. If that is not available to you, you can test it manually using a pot of water, a candy thermometer, and an ohm meter. You will need to constantly monitor using the chart in the following link to make sure the resistance at all temp ranges falls into the proper area.