There are several sending units on that vehicle (coolant temp, oil pressure, fuel level), need to know which one you are asking about.
In the fuel tank. Fuel tank must be removed to gain access to the sending unit.
Locating on a 2000 s10 pickup truck: bronzer idea
With a 4.3L engine, the oil pressure sending unit is in the back of the engine just to the drivers side of the distributor.
The fuel level sending unit in the tank is probably at fault.
It is mounted on the top of the fuel sending unit that goes in the TOP of the fuel tank.
how do you replace the fuel sending unit
It is on the fuel sending unit and that is what goes in the top of the fuel tank. You will need to drop the tank are raise the bed up to get to it.
The fuel tank has to be removed to gain access to the fuel sending unit. The fuel sending unit and the fuel pump is replaced as a unit.
In the fuel tank. The fuel tank needs to be removed to gain access to the sending unit.
Next to the thermostat housing.
Assuming you mean the engine coolant temperature sending unit, we need to know what engine it has, it makes a difference.
If you suspect the sending unit may be faulty just replace it with a new one, they are not expensive but a little difficult to remove and replace.