A computer starts to boot then quickly shuts down. Which of the following should the technician use to troubleshoot this issue?

A toner probe is used to trace cable runs through walls and ceilings and has no application in diagnosing power or boot issues.

A power supply tester connects directly to the PSU connectors and measures the voltage output on each rail (3.3V, 5V, 12V), quickly revealing whether the power supply is delivering stable and sufficient power. Unexpected shutdowns shortly after power-on are a classic symptom of an underpowered, failing, or overloaded PSU that cannot sustain the load as components initialize. This is a fast, non-destructive way to confirm or rule out the power supply as the root cause.

Loopback plugs test network port functionality and have no relevance to diagnosing a computer that shuts down during boot.

Event logs require the system to successfully boot far enough to log events; if the computer shuts down too quickly, no useful log entries will be recorded.