"I replaced a post light sensor from HD (westek w/ black, red, white) according to the diagram and it still does not work"

So, why did you replace it? What happened when you originally troubleshot the problem and bypassed the sensor by connecting the line and load directly to confirm that the sensor was at fault?

Ok, you got me. I didn't. The eye looked cloudy/rusty and was missing the gasket and ring nut so I jumped to conclusions and just replaced it. My bad.

I'll be back monday to figure it out. (It'll probably turn out to be the socket after all.)

"There's always current (110) at the socket whether the sensor is blacked out or not. Do these things switch the hot or the neutral to the socket?"

No, there is voltage.

The current may be so low the light is off.

This is common with a digital meter and a solid state switch.
There is enough leakage of current (even micro-amps) for a digital meter (10,000 mega ohms input impedance on the voltage scale) is not hard to find on a digital meter) to read a voltage when you cannot even feel the tiny current.

Hots are switched.

Update:

Correct that there was voltage but no current. Also, the lamppost was NOT wired with UF cable and there apparently is no conduit underground either. Nice digging project for when storm "Sandy" is long gone.

If you use rigid metal or intermediate metal conduit the coverage depth is only 6 inches.

A single shovel depth trench does the job.

Use a 90 degree sweep turn with a thread protector on the open end to curve up into the post.