How it works
Power-on self-test is an early hardware-initialisation and checking phase before the operating system starts. It can expose errors through screen messages, indicator patterns or sounds. The codes are specific to the manufacturer's design and sometimes the model family. A beep pattern from one computer should not be interpreted using a random table for another. Passing POST also does not mean every component has undergone a comprehensive diagnostic test.
Distinguish no power, no POST, no boot and no visible image. A computer can have working power lights but fail during initialisation, or complete initialisation and then fail to locate the operating system. A disconnected monitor can hide useful information without preventing all startup activity. This distinction helps select the right investigation rather than immediately reinstalling software for an earlier hardware-stage problem.
Practical checks
Record the exact light sequence, sounds and message before repeatedly changing parts. Note recent hardware installation or firmware changes. Consult the model's service documentation and use its supported diagnostic steps. Disconnect only the accessories the procedure identifies and observe whether the pattern changes. Avoid repeated forced interruptions during firmware recovery. If component work is required, shut down and follow the specified power-isolation procedure; preserve the original evidence for repair diagnosis.
Additional examples and checks
Fans may spin and lights turn on while the system fails an initial memory check. That is different from a machine that completes startup checks but cannot load its operating system. Distinguishing the stage narrows the investigation.
Record indicators precisely, including timing and repetition, and consult the model’s diagnostic guide. Note recent component changes. Check only safe external basics first; internal reseating requires power disconnection and the specified service precautions.