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Power-on self-test (POST) is a set of checks a computer performs after it is powered on and before its operating system loads. It checks selected hardware and configuration items; if it detects a problem, startup may stop and the computer may show a message, beep, or diagnostic light.
What POST means
POST stands for power-on self-test. Dell Support defines it as “a series of checks the computer goes through whenever it starts” (Dell Support). It is part of the startup process, not a separate application or a test you normally need to launch yourself.
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POST runs after power-on and before the operating system loads, as IBM describes in its overview of POST.
What POST checks
The checks vary by computer, firmware, and configuration; there is no universal list of tests every system performs. As examples for a particular IBM IntelliStation M Pro Type 9229 and its era, IBM’s service guide describes checking basic system-board operations, memory operation, video startup, and installed drives (IBM service guide). Those examples should not be treated as a checklist for all current computers.
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What happens if POST finds a problem
A detected fault can prevent the computer from continuing to load the operating system. Depending on the model, the system may show an on-screen error, emit a beep code, or display a diagnostic light pattern. IBM documents beep codes and messages; Dell describes diagnostic LEDs for startup problems (Dell’s POST guidance; IBM service guide).
These signals are model-specific, not universal translations. Find the documentation for the exact computer or motherboard before interpreting a beep sequence or light pattern. Dell’s troubleshooting material also distinguishes symptoms such as no power, POST failure, boot failure, and no video, which can look similar but occur at different stages (Dell startup troubleshooting).
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What a successful POST does—and does not—tell you
Completing POST means the system passed the checks it performed well enough to continue startup. It does not prove that every component is healthy or rule out an intermittent fault. IBM distinguishes POST indications from separate diagnostic programs, which are the primary method for testing major components in the system covered by its documentation (IBM service guide).
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POST versus boot failure
POST happens before the operating system loads. If the computer reports a POST error or halts during that early startup stage, use the model-specific indicator and manufacturer guidance to investigate. If POST completes but the operating system does not start, the problem is at a later stage; a blank screen alone does not establish which stage failed. Distinguish the symptom and stage before choosing troubleshooting steps.
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