I replaced a coolant sensor recently (the one that screws into the block aft of the coolant bleed screw), because the previous one was leaking coolant.

Unfortunately, the leak persists. Questions-

1-What is the sensor for?

2- Does this sensor require a sealing ring or washer to stop the leak?

3-Will applying a high temperature-resistant sealant to the threads impair the functioning of the sensor?

4-In short, what is the best way to stop the leak?

The leak is slow, but is enough to annoy. Thanks in advance for your advice.