What they're trying to fix with the harness replacement is the failure of the High Voltage Interlock pins in the main battery feed connector. There is really nothing wrong with the harness, but the little shorting pins are breaking their connections and causing the car to think you unplugged a high voltage connector. There are two HVIL circuits in the car. One is in the battery module. The other is in the TCM and is under the hood. The focus of the harness replacement is to try and correct the issue of the HVIL opening up. The HVIL is made up of one continuous circuit completed by pins in all of the high voltage connectors and the lid to the TCM. Should one of these pins not sit right, or develop some corrosion, the circuit will intermittently open and then cause the car to shut down. The reason it always comes right back after a reboot is that the car now sees the HVIL is closed and you're good to go. This doesn't set a persistent code. I've attached a photo of inside of the TCM. The green wires are the HVIL circuit. Note they go to all the connectors that are on the TCM. Note how small they are, and how they didn't even give enough slack for these not to be strained. So, unless there is a resistance to frame or line to line fault being caused (most likely not, or you wouldn't be able to restart your cars), they are replacing a $1k+ harness for $0.02 of shorting pins... TE connectivity (the manufacturer of these big orange connectors) doesn't even make this particular type any more except for Ford. It's an old model, and there are better ones out there now. TSB 15-0031 addresses cleaning these pins. I would have the dealer try that if you're having SSN issues.