POST (Power On Self Test)
The diagnostic routine your laptop runs every time it powers on. A POST failure means the laptop cannot complete startup — and it’s always a hardware fault, not a software problem.
Power On Self Test (POST) is a firmware-level diagnostic that runs automatically every time a laptop is powered on. In a fraction of a second, the firmware checks that the CPU, RAM, storage, GPU, and other critical hardware are present and responding correctly before handing control to the operating system.
If any check fails, the laptop halts — it cannot proceed to load the operating system. This is POST failure. The result is a laptop that appears to power on (fans spin, keyboard lights up) but shows a black screen or error message and never reaches Windows or macOS.
- Laptop powers on but shows a completely black screen — no manufacturer logo
- Beep codes on startup (a sequence of beeps indicating which check failed)
- Flashing LED indicators in a specific pattern (manufacturer error codes)
- Error message on screen: “No bootable device”, “Memory error”, or similar
- Laptop powers on briefly then immediately shuts off
| Cause | What Fails POST Check |
|---|---|
| Failed RAM | Memory check |
| GPU failure | Graphics output check |
| Failed storage drive | Storage device check |
| Corrupted BIOS | Firmware integrity check |
| Liquid damage to board | Multiple checks simultaneously |
Can POST failure be fixed by reinstalling Windows? +
No — POST failure occurs before the operating system loads. It is entirely a hardware problem. Reinstalling Windows, resetting the laptop, or any software fix will have no effect on a POST failure. The faulty hardware component must be identified and repaired.
How is POST failure diagnosed? +
Technicians use a combination of beep codes, LED flash patterns, and specialist diagnostic tools (POST code readers, multimeters, oscilloscopes) to identify which hardware check is failing. The diagnosis process is systematic — ruling out each component until the fault is isolated.
Is a POST failure always serious? +
Not necessarily — some POST failures have simple causes (a loose RAM stick, a disconnected storage drive after a drop) that are easily fixed. Others indicate more serious hardware failures (GPU fault, liquid damage to the board). Diagnosis determines severity and repair cost.
My laptop turns on but won’t boot — is that a POST failure? +
Possibly — but “turns on but won’t boot” can also mean the OS is corrupted (still a software problem). The key distinction: if you see the manufacturer logo briefly before failing, the OS is likely the issue. If you see nothing at all (black screen from the moment of power-on), POST failure is more likely.
Free board-level diagnosis to find the exact cause. Component-level repair where possible.
