Applies to most other brands of front-loaders, too:

<<<You mention the drain motor may run too slow, how can I troubleshoot that part?>>>

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A simple test---would be to start a cycle and wait until water stops entering/filling the washer (about 1 minute)

Turn off the washer

Turn on the washer

Select SPIN SPEED to "No Spin" and press the START/PAUSE button

Observe the water draining from the Drain Hose.

The flow-rate should be solid (but not overly agressive either).

When DRAIN MOTORS begin to fail---the flow rate of water draining appears somewhat anemic.

This is *usually* in addition to the Drain Motor becoming noisier than it was previously.

If needed---the DRAIN MOTOR part number is...

DRAIN MOTOR : 4681EA2001D

<<<there was about 1 cup of water still left in the tub>>>

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If this water is seen in the *stainless steel tub* and not in the Pump Housing when the *strainer* is removed---replace the DRAIN MOTOR (some water must remain in the housing and is normal).

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Source: LG WM3988HWA Does Not Dry