Hall effect sensor and pull down resistor

Do I need an external pulldown resistor in order to get correct readings from a hall effect sensor?

I'm using a Mega2560 and an DC NPN NO 5V inductive proximity sensor

NPN means a switch to ground, so you need a pull up resistor.

The Mega has buildin pull up that you can activate with code.
No physical/external resistor resistor needed.

pinMode(sensorPin, INPUT_PULLUP);

Leo..

Pull UP (not: Pull Down) is often used to force the logic level high when the sensor is not active.

Search your sensor datasheet for more details.

Wiring a NPN (Sinking) switch sensor operating above 5V (DC)

http://qqtrading.com.my/proximity-induction-sensor-npn-normally-open-switch-lj12a3-4

NO (normally open) could mean digital, so no need to use an analogue pin.
Always best to post a link to the datasheet of the sensor, so we don't have to guess.
Leo..