I can't find if there's a difference between model PLMV178HC and PLMV178HCA. Every search for PLMV178HC redirects me to PLMV178HCA.
Unless there is grease or moisture on the control circuit board, then you have a shorting membrane keypad.
It is repaired by replacing the control panel frame assembly, which comes with a new membrane keypad already attached.
For model PLMV178HCA, that is part number 5304470533
and you can get it from Sears
online for $89.95 (subject to change).
At my Web site
, I have a video available showing how to remove a typical over the range control panel assembly in under 5 minutes. (Also, the service manual is here
If you have any trouble getting the grille off once the screws are removed, slide the hood vent grille to the left about 1/4", then then pull it forward. On some models there are L-shaped clips that fit into a slot to hold bottom of the vent.
Be very careful when disconnecting the cables & wires. Some connectors have a locking tab (see photo) Next remove the keypad strip (see photo) Then simply move your circuit board into the new frame.
Be careful, because your display tube is made of glass, and if it breaks, the board must be replaced as well.
Let me know by e-mail if I can help further.
William E. Miller, AS-EET
Microwave Oven Control Panel Repair Nationwide "Recycling by repairing since 1982"