Isolating power supply issue to USB-C cable

Is there a way to test for faults (power supply and/or data transfer) in a possibly failing USB-C cable, by plugging each end into different ports on the same machine (2018 MBP) and sending traffic from one port to the other?

Unfortunately, no. There is nothing that operates at the (using the OSI model as an analogy) application layer that will diagnose the physical layer connections. To properly diagnose a cable, you need to, at minimum, use a multimeter to check for continuity and resistance on each conductor in the cable. A better option would be to use an oscilloscope to check for noise and signal quality.

However, attempting to “break out” the individual conductors to get a probe tip in there (as well as the cost of the diagnostic equipment) far outstrips the idea of simply getting a known working cable and seeing if it solves your problem. This should’ve been your first action.

My money is on the USB-C cable.... USB-C is the flimsiest of the cables, and gets the most abuse, so it's the prime suspect.

Yes, I agree. The easiest (and most effective) solution is to replace it with a known working cable. I personally don’t use the cables included with my Apple devices because of their low quality. Instead I get high quality reinforced (usually braided) cables like Anker USB-C cables.

Could it be the ports and/or logic board? Possibly, but again, you can diagnose with items you probably have on hand.

• Try a different port, especially on the opposite side. The 2018 MBP has 4 ports, meaning separate USB controllers and power rails for each side. If the results are the same, look to the charger/cable
• Try a known working charger and cable. If the symptoms persist, it’s likely the computer
• Try plugging in an accessory device like a flash disk. If it works, your port is physically good.
• Try charging/powering a downstream device (like a phone or even another USB-C Mac). If it works, it tells you that the port is physically good and that the USB controller and power rail (for that side) are functioning. The port wouldn’t be able to negotiate and deliver power otherwise.

As for diagnosing your USB-C charger, this is even easier; it’s not limited to charging Macs or even solely Apple products in particular. Just try charging another device like a phone or tablet. It’s ideal if you can test with a known working cable, but you can narrow down the issue to “the charger and cable” with what you have in your possession by attempting to charge something different.