I am not familiar with a sensor that you are supposed to put in the radiator hose. Is that where the instructions said to install the sensor? It seems odd to actually put in in the hose. All I have seen are either shoved in between the fins in the radiator or wired into an engine block mounted sensor.

Could you possibly have the fan turning the wrong direction? Reversed polarity will push air through the radiator instead of pulling it through. If the controller is not powered by a switch controlled wire the fan will run until the water is cool according to your temp setting. You could have one relay running power to the other so that the fans only have power when the key switch is on if you are worried about it draining the battery when the engine isn't running. This is only needed if you are using the Bosch type relay. I had a GrandAm that would run the fans after the engine shut down and would not shut off until the water temp came down. When you shut the engine off, heat soak starts and all the heat from the block boils the water for a while and that will raise the water temp. If the fan controller isn't key switch controlled, the fans should run until the water temp subsides.