My desktop computer turns on for a brief moment but then shuts down immediately. What should I do?

You’ve just built your new PC and now it doesn’t start. You press the power button, the fans spin for a second, and then PC turns off.  

The first thing you should do is check the debug LED lights on the motherboard. They are super helpful in these types of cases. 

During the POST stage the LEDs indicate which components may have problems. Whenever an error occurs, the corresponding LED stays lit until the problem is resolved.

To understand each error code you have to consult with the motherboard manual.

For example MSI B450 Tomahawk MAX II has four debug LEDs – CPU, DRAM, VGA, BOOT.  When lit, these LEDs indicate that the corresponding component has failed or was not detected.

Another thing to consider is the sound (beep) error codes. They are available if your motherboard has a built-in speaker. Again, you have to check the manual for corresponding error code.

In our Tomahawk example, there are three long beeps for memory errors and one long followed by two short beeps for graphics card issues.

To resolve the “my computer starts just for a brief second” issue, it is necessary to open the case and execute the following suggestion in the order.

Take your PSU cables and connect only the motherboard power, the CPU power, and the CPU cooler/fan.

Disconnect everything else – RAM, GPUs, SSDs, everything.

Clear the CMOS by removing the battery for 15 seconds.

Step one: Checking the CPU

It is a good idea to find out whether the motherboard supports the processor you have. For example you may need a BIOS update if you have an older motherboard and new CPU, something like a Ryzen 5000 CPU on a B450 chipset motherboard.

Turn on the computer and listen for a beep or check the debug light. If there is no beep and the PC still shuts down then you may have bad power supply, processor or motherboard. In this case you have to check with the repair shop to investigate further or start swapping parts.

If it beeps and the fans keep spinning then you can continue with the next step.

Step two: Checking the RAM

Now we start introducing PC components one by one.

Turn off the desktop computer and insert just a single memory module. Turn back on – did you hear any beeps? If there are no beeps or lights, check if the RAM was placed properly. 

If you hear the same beeps as before or RAM debug light is on – turn off the PC and swap the current memory module with another one. If the beep or the light changes the previously tested  RAM stick was bad and you should return the whole kit or replace the fault module. 

Now when the light or beep error code has changed, you can continue to the next step.  

Step three: Testing the graphics card

At this point if your computer has a CPU with integrated graphics card, it may boot. So make sure when you are debugging with an integrated GPU, to plug your monitor to the connector located on the motherboard.

If you don’t have such a CPU or don’t want to use the integrated GPU, turn off the PC and install the dedicated graphics card with all the necessary power connectors. If the system beeps or the GPU debug light is on then your graphics card is not connected properly or busted.

Here, the best approach is to test with a different GPU or use the integrated one (if the CPU has one) to boot your PC.