Can't get cordless Earthworks mower to run.

I have a Greenworks 40V, 19" lawn mower. It won't run, and I have torn it apart and tested everything, including the motor (runs great when power is hooked directly to it), the 'kill' switch (the one that activates when you release the bail on the handle), the safety key circuit, all of the wires, connectors etc. are in good shape (metered them out). The battery is fully charged.

However, there is a small, black, square module that appears to be a heat sink which connects to the motor. I'm not sure what it is and what it does, so I can't really test it. Since it's the only thing left that I haven't tested, I'm sure it must be the root of the problem. The owner's manual does not have a wiring diagram (why do they always omit those?), and the parts list & 'exploded' view of the mower do not show this small device, even though it shows every single other parts, including even screws. When I took the device apart and looked, I could see what appears to be some major heat-sink components. I'm thinking that maybe this device, whatever it is, may have been an often-faulty one, so they have now redesigned the mower, and are now not even including it in their manuals. The manual I was looking at was one I found online, and the mower is my son's, so I don't have the actual manual that came with the mower.

Any help would be much appreciated.