What is the sensor on the drivers side between number 3 and 5 piston on a 4.3

That is the coolant temperature switch or sending unit, it is for the temperature gauge..one wire connected... The coolant temperature 'sensor' is right next to the thermostat housing on intake manifold, it tells the ECM what engine temperature is for fuel adjustments..two wires connected... Both work basically the same but have two different purposes... In the later year models, the 'sensor' services both purposes.

It's a coolant sensor. Mine slides in and out. With it unplugged my temp gauge doesn't read anything. Replacing mine tomorrow