The Water Level Frequency Sensor operation is based on the displacement of the *coil* attached to the diaphragm.
When the *coil* raises or lowers---it changes the electronically resonant characteristics of the oscillator circuit.
The water level is measured by the Main Board---by frequency readings.
I've never had a failed Water Level Sensor.
It is best tested while on the washer---by using the TEST MODE.
On most LG washers---the Test Mode is enabled by pressing and holding the SPIN SPEED and SOIL LEVEL buttons---and then pressing the POWER button (release all 3 buttons and wait for the door to lock).
Press the START / PAUSE button 4 times at one second intervals (Prewash Cold Water Fill)
The display read-out on the Control Panel will show the *frequency*
Empty---the frequency read-out should be about 255
A tub full of water should show a frequency of about 214 (depending on model number)
Some of the errors that can be displayed---relating to water fill are...
Little or no water---usually caused by a failed Water Valve
Too much water (overflow detected)---Can be a failed/stuck open Water Valve or blockage in the black air tube which is connected to the Water Level Frequency Sensor. Disconnect the tube from the sensor---and blow air back into the tub to clear obstruction.
To test the Water Level Frequency Sensor outside of the washer...
Use a test meter to check the ohms between pins 1 and 3 (should have between 21 & 23 ohms).
Source: LG PRESSURE SENSOR OPERATION