Pretty much all the brain stuff runs off 5v even if indirectly, and any external stuff you have like throttles, motor halls, or other sensors run from 5v provided by the controller.
So any of those things could drag 5v down. If it's a direct short, zero ohms, it's more likely to be a wiring fault than a component fault, but you'd have to open it up to test all the parts on the 5v bus to find out whihc one causes it. That requires disconnecting parts on the 5v bus until the resistance goes back to normal.
Even after you find and replace the failed part, the actual 5v supply electronics inside could be damaged and still not supply 5v until replaced.
So any of those things could drag 5v down. If it's a direct short, zero ohms, it's more likely to be a wiring fault than a component fault, but you'd have to open it up to test all the parts on the 5v bus to find out whihc one causes it. That requires disconnecting parts on the 5v bus until the resistance goes back to normal.
Even after you find and replace the failed part, the actual 5v supply electronics inside could be damaged and still not supply 5v until replaced.
