Coolant leak

Coolant temperature sensors rarely leak, and when they do, it is just a little see page through the electrical connector. If you think that is what is leaking, get the new one first so you can look at it and see which socket is needed to remove and reinstall it. If it is a two-wire sensor, that is for the Engine Computer. Do not turn on the ignition switch while that sensor is unplugged. That way no diagnostic fault code will be set and the Check Engine light will not turn on.

I like to use Teflon tape to seal the threads to insure against seepage, but on single-wire sensors, one of the terminals is those threads. Teflon tape will insulate the threads and prevent the sensor from working. That usually only applies to sensors for dash gauges.

If the sensor is screwed into the thermostat housing, over-tightening it can crack the housing, then you will really have a leak. Most coolant sensors screw into the intake manifold or side of the engine block. Those are much tougher. If the sensor is down below to top of the engine, remember that leaking coolant will run down. The sensor might be wet, but look if there is coolant up higher that is running down to it. The source of the leak is the highest point you see coolant.