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Troubleshooting a PC that won't POST: lights, beeps and common causes

TUREFF• •0
POST, the power-on self-test, is the check a motherboard runs each time it starts. A PC that will not POST may spin its fans but never show a picture, or may not start at all. It is frustrating, especially on a brand-new build, but the cause is usually a cable or seating problem rather than a dead part. Work through this methodically, one change at a time.

First, wait

A new DDR5 build, or any build after a BIOS update or memory change, performs memory training on the first start. The screen can stay black for several minutes and the system may restart itself. Give it at least a few minutes before deciding something is wrong.

Read the debug lights

Most modern motherboards have four small lights near the memory slots, often labelled CPU, DRAM, VGA and BOOT. During a normal start they light briefly in turn and then go out. The one that stays lit tells you roughly where the start-up stopped.
  • CPU: the processor is not being initialised. Check the CPU power cable, the CPU seating, bent socket pins and whether the BIOS supports this CPU.
  • DRAM: memory is not initialising. This is one of the commonest problems. Reseat the sticks, use the recommended slots and try one stick at a time.
  • VGA: no graphics was found. Check the graphics card is seated and its power cables are connected, and that the monitor is plugged into the card.
  • BOOT: the hardware checks passed but no bootable drive was found. This is usually a drive or Windows installation issue rather than a fault with the motherboard.
Some boards also show a two-digit code display or use beeps if a speaker is connected. Look up the code or beep pattern in your motherboard manual, as they differ between makers.

The common causes, in order

  1. Monitor in the wrong port. If your CPU has integrated graphics, the motherboard ports may be active but the graphics card is the one in use. Plug the monitor into the card.
  2. CPU power cable missing or loose. The cable at the top left of the board is easy to forget. Some boards have two sockets; check the manual for which are required.
  3. Memory not fully seated. DDR5 sticks need firm pressure and both clips should click. Reseat them.
  4. Graphics card power not connected or a connector not fully pushed in.
  5. Front panel power switch wires on the wrong pins, so the button does nothing.
  6. The BIOS does not support the CPU. A board made before the processor was released may need an update. If the board has a BIOS flashback button, you can update it from a USB stick without a working CPU.
  7. XMP or EXPO profile unstable. If it stopped booting after enabling a profile, clear the CMOS to return to defaults.

Clearing the CMOS

This resets the BIOS to default settings and often fixes no-POST problems after a settings change.
  • Use the clear CMOS button on the rear panel, if your board has one.
  • Or use the jumper described in the manual.
  • Or remove the coin battery for a few minutes with the power supply switched off at the back.

Minimum configuration test

If nothing above helps, strip the system back to the bare minimum:
  1. Switch off and unplug the power supply.
  2. Leave only the CPU, cooler, one memory stick and the power supply connected. Use integrated graphics if the CPU has it, otherwise the graphics card.
  3. Disconnect all drives, extra fans and USB devices.
  4. Try starting. If it works, add parts back one by one until the fault returns.
  5. Try the single memory stick in a different recommended slot, and then the other stick.

Physical checks

  • Bent pins. Current Intel and AMD AM5 boards have the pins in the motherboard socket, while older AM4 processors carry them on the CPU. Inspect with a torch for any out of line.
  • Overtightened cooler pressing unevenly on the CPU.
  • A stray screw or standoff shorting the back of the board.
  • The power supply switch at the back turned off, or a faulty power cable.

When to seek help

If the minimum configuration still will not POST, the likely suspects are the motherboard, CPU or power supply. Testing parts in another system, or asking the retailer or a repair shop to test them, is the next step, and most components have a warranty for exactly this. What fixed your no-POST problem in the end? Share it in the comments to help the next person.

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