Skip the engineer, I'd try a kid who plays with computers.
I screw mine up all the time and the last issue was a fine hairline crack that interupted the welded connection between the circuit board and my battery connection. The vibrations from the electric wheelbarrow could easily break soldering and it is very tricky to see. It occurred to me, that if you took a digital picture, it would be easier to magnify and look for the problem.
Thanks for the suggestion - but by a clear fluke, my guy accidentally figured a fix. Stuck a screwdriver somewhere or other and it took off.
I have absolutely no idea what he meant when he described how he fixed it; to me it made no sense at all, but I'm glad it's working
So, do you have the same machine and have you ever needed parts? We can get a battery local, but also need a new back wheel soon. I just can't imagine they are no longer on the market. It's such a handy thing to have for hauling.