A mail server keeps shutting down. The administrator gets an error message over temperature error. Which of the following should the administrator replace FIRST?

While a power supply fan cools the PSU, a general 'over temperature error' usually points to the main system or CPU temperature, not specifically the PSU.

The CPU fan is specifically designed to cool the Central Processing Unit, which generates significant heat and is vital for server operation. An 'over temperature error' leading to system shutdown is a strong indicator that the CPU is reaching critical temperatures due to inadequate cooling, making its dedicated fan the most likely component to have failed and need replacement first.

A heat sink dissipates heat but requires a functional fan to effectively remove that heat from the CPU; the heat sink itself rarely fails in a way that causes an over-temperature shutdown unless it's improperly seated or clogged.

A case fan improves overall airflow but a specific 'over temperature error' causing a shutdown is typically triggered by a critical component like the CPU, which is primarily cooled by its dedicated fan.