OK Martin, update. I removed the 2-wire harness between the Z-stop switch and the 4.2.7 board, and checked continuity of both leads with a multi-meter, and both were good. I'm not sure how to check within the plug sockets themselves, so that remains as a potential problem. I re-downloaded the .Zip file from creality.com/download that you suggested. Unzipped, several options appeared, like for BL, transfer and filament boards, and I selected ORIGINAL FIRMWARE. Next, 3 options were available--(1) Ender-3 Pro 4.2.7 mainboard firmware (32 bit) (2) Ender-3 Pro Marlin2.0.1 V1.0.1 TMC2225 English (3) A file similar to #2 except in Chinese. I chose #2 (#1 was not a .bin file). Copied that file to a blank formatted 8 GB SD card. Put card into printer and turned it on. A blue screen appeared which shortly became the usual Creality logo, but with no other response other than an occasional blink and a high pitched whine. So my nightmare continues. If you were in the US, I'd presumptuously suggest using a remote control app like Team Viewer, just to get me out of the loop because I must somehow be the problem!