pi 2 – Raspberry Pi undervoltage strange issues

So I have an old Rpi2 B, and I was getting an undervoltage error trying to run OctoPrint on an old bundled PSU.

I swapped for a 5V/4A PSU with the same error, I only have the power supply and an Ethernet cable connected. Could my Pi be broken in some way?

I took it a step further and ran 5.15V on the GPIO pins from a decent lab PSU. (I know it’s risky, but it’s a last resort.)
This drew around 400 mA and still had the same error. I then tested the voltage out of the USB sockets and found around 4.8 v.

My theory was that plugging in my 3D printer would pull enough current to under-volt the PI; however, even without the printer connected, the issue still occurs, so even that theory is out the window.