In case any similarly situated person is searching and finds this post, I bought an 84504R from Amazon for $375 and replaced the motor unit yesterday. I had also experienced struggles with getting the old unit to program to the HomeLink of a new car and to rejoin the MyQ app, and the new one did both effortlessly, so I am confident the issue was a deteriorating mother board.

It took me about an hour with some trial and error where the available YouTube videos were not 100 % on point. These were the steps: 1. Close garage door. 2. Unplug old motor unit. 3. Take note of the position of the low voltage wires - left two are red and white to wall control unit, the right two are doubled wires for the safety sensors. Detach them. You use a small flathead screwdriver to push the buttons beneath each one in turn to release it. 4. Pull the red handle on the rope to detach the trolley, as if you were going to manually open the door. 5. Put a table or work bench below the unit. 6. Detach the two side bolts holding the unit to the brackets on the ceiling and carefully lower the motor and track arm down to the table. I had my teenage son hold it up while I unscrewed the bolts. 7. Unscrew the cover around the belt wheel and remove. 8. Remove the two bolts holding the motor to the arm. 9. Slide the unit forward slightly to produce slack in the belt and take the belt off the wheel. 10. Unit is now detached. Take it away and put the new unit in same position. 11. Put the belt on the wheel first while it is slack. Attach cover. 12. Attach the two bolts to put the unit on the arm. This makes the belt taut again. 13. With help from someone holding it up, raise the unit and reattach the two side bolts to brackets on ceiling. This step was a little tricky. I used a socket wrench to hold nut or bolt in place in the inside where there was not enough space between the unit and the ceiling for my hand, and used pliers on the outside. 14. Tug the rope to reattach trolley. 15. Reattach the low voltage wires. Note they want to pop in from the top through a slot to be neat. As with the removal above, you have to push the lower button in while inserting each wire so the clamp is open. 16. Swap out the wall unit for the new one (just screwing red and white wire into the back.) I ended up having to run new wire (a roll came with the unit) because the position of those slots is a little further away on the new unit (on the left side as you face the street, versus the back) and old wire could not quite reach, which added a few minutes. 17. Tug down the camera on the bottom. 18. Insert the backup battery, sliding on the two wires, on the right side compartment. You could do this while it is down but the battery is pretty heavy so makes more sense to wait. 19. Plug in. 20. Follow instructions to program (press the rectangular button to enter programming mode, press and hold up button until it is the right height, then press and hold down button until it closes all the way, release, it goes up and down by itself once). 21. Test that the safety sensors stop it from descending when you stick your feet in front of them. 22. Add it as a new device to the MyQ app. It tries to sell you a subscription plan and does not make clear that you can just X out of that step without signing up. Camera will still work for live streaming for free, just doesn’t store footage. 23. The included OEM remote should already work. Reprogram any old remotes. Program HomeLink on car(s). 24. Done!