1st off....
Have you checked.verified that the 120V AC Outlet fridge plugs into is HOT?
If Hot then need to check that circuit board is getting AC Voltage
J5 is Line (Hot--Black wire)
J6 is Neutral (White wire)
Then verify that the 5A glass fuse (120V AC) is functioning (test it....not just visual) and fuse clips also
J7 & J8 are heater leads (black wires)
GREEN wire & Green/Yellow stripe is 120V AC ground
Got AC Voltage but still swaps to LP.....that would be a circuit board issue not heat element
You can test heat element by removing leads from circuit board and plugging them directly into a working outlet
(tests/verifies element works or not and bypasses circuit board)
After a few minutes you can tell whether or not element gets hot
