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PC Won't Turn On? Work Through It in This Order
Short answer: Work through it in this order: confirm the PC is getting mains power, disconnect every peripheral except one monitor, then reseat the RAM, then the GPU and its power cables, then the M.2 drive, then the 24-pin and 12V CPU power connectors. RAM is the single most common cause of a system that will not complete POST, so start there once power is confirmed.
A PC that will not start covers two different situations: no signs of life at all, and a system that powers up with fans spinning and lights on but never reaches the point of showing you anything. Both usually come down to the same short list of causes, and both are worked through the same way.
What is actually failing
When a PC starts correctly it runs a Power On Self Test, or POST, checking for hardware faults before handing over to your operating system. If POST fails, the system either hangs or powers itself back off. That is what you are diagnosing.
The usual causes, in rough order of how often they turn up:
- Loose or improperly seated hardware, most often RAM or the graphics card
- Loose power connections, particularly the 12V CPU, 24-pin ATX or PCIe/GPU power
- A connected USB device or other peripheral interfering with startup
- Genuinely defective hardware
On most systems this takes a few minutes to work through. Go in order and do not skip steps, because skipping is how people end up replacing a working part.
Step 1: Confirm the PC is getting power
Obvious, and worth doing properly anyway.
- Check the power cable is firmly seated at the back of the power supply. These work loose more often than you would think.
- Confirm the outlet has power. If you are on a plug strip or surge protector, confirm the strip itself is live and switched on.
- If the PC shows no signs of life at all, try an outlet in a different room. This rules out the circuit as well as the socket.
- Check the power switch on the back of the PSU is set to the I position.
Step 2: Strip it back to the essentials
Disconnect every USB and external device except a single monitor. A faulty peripheral can hold up POST, and it is a five second test.
Make sure the monitor is connected to your graphics card, not to the motherboard's video output. On a system with a dedicated GPU, the card's outputs sit lower on the back of the case than the motherboard's rear ports. Plugging into the motherboard on a system with a dedicated GPU is one of the most common reasons for a "dead" PC that is running perfectly well.
Now try starting it.
- No signs of power at all: continue to the hardware steps below.
- Powers on and boots into Windows: the problem was a peripheral. Shut down, reconnect devices one at a time and test after each, until you find the one that causes it.
- Powers on but no video and no Windows: continue to the hardware steps below.
Step 3: Reseat the RAM
RAM is the most common reason a system fails to complete POST. A stick can work loose from nothing more than the machine being moved or vibrating over time.
If your motherboard has debug features, use them first. A two-digit code readout or a set of four debug LEDs will tell you where it is stopping. A lit DRAM LED, or a code generally in the 40 to 5F range, points straight at memory.
Power the system down fully and disconnect it from the wall, then:
- Release the clips and remove each stick of RAM.
- Reseat them firmly until the clips click closed on their own.
- Try to start the PC.
If it still will not POST, remove all the RAM and test one stick at a time, starting in the fourth slot (the one furthest from the CPU). Power off completely between every change. This tells you whether a single stick or a single slot is at fault.
Step 4: Reseat the graphics card and its power
The GPU is the second most common cause, either because the card has worked loose or because it is not getting enough power.
With the PC fully powered off and disconnected from the wall:
- Release the PCIe slot retention clip and remove the card.
- Reseat it firmly until the clip engages.
- Reseat the PCIe power connectors at the card.
- Follow each power cable back to the power supply and confirm it is fully seated there too. This end gets overlooked constantly.
- Reconnect the monitor to the card, with the system powered off.
Step 5: Check the M.2 drive
An M.2 NVMe drive that is loose or badly seated can short, which stops the system starting or completing POST.
Reseat the drive, making sure it sits flat and the retention screw or latch is holding it down. Check underneath the graphics card while you are in there, because some boards put additional M.2 slots there and it is easy to forget one exists.
If the system still will not POST, try starting it with the M.2 drive removed entirely. If it reaches the UEFI BIOS without the drive installed, you have found your fault.
Step 6: Check the main power connectors
Two connectors matter most here:
- 12V CPU power, normally at the top left of the motherboard. A system with this disconnected will often show signs of life and never POST, which is exactly the symptom that sends people chasing the wrong part.
- 24-pin ATX power, along the right edge of the motherboard.
Both use a locking clip, so it is easy to feel whether they are properly home. Unclip and reseat each one to be certain.
If it still will not POST
At this point you have ruled out the common causes, and you are looking at either a genuinely faulty component or something that needs deeper diagnosis. Clearing the CMOS is a reasonable next step, particularly if the system was working before a BIOS change or a hardware swap. Our guide to clearing CMOS covers three methods.
If you have an NZXT prebuilt PC that will not POST after working through this guide, contact NZXT Support. All NZXT prebuilt PCs carry a standard two-year limited warranty.
Frequently asked questions
My fans spin but nothing appears on screen. Is the PC on?
It is powered, but it is not completing POST. Fans spinning only tells you the PSU is delivering power. Work through the RAM and GPU steps above, and confirm the monitor is plugged into the graphics card rather than the motherboard.
What are the debug LEDs on my motherboard?
Many boards have four LEDs labelled CPU, DRAM, VGA and BOOT. The one that stays lit shows where POST stopped. It is the fastest diagnostic you have, so check it before you take anything apart.
Which RAM slot should I test a single stick in?
Start with the fourth slot, furthest from the CPU. Power the system down fully between every change.
Could a faulty keyboard or USB hub stop my PC booting?
Yes. A misbehaving USB device can hold up POST. This is why the first step is to disconnect everything except one monitor, then reintroduce devices one at a time.
My PC turns on for a second then shuts off, repeatedly. What is that?
A short power-cycle loop usually points to a power delivery problem or a hardware fault detected during POST. Check the 12V CPU and 24-pin connectors first, then reseat RAM and GPU. If it persists, get in touch with support.
Do I need to clear the CMOS before doing all this?
Not first. Confirm power, strip the peripherals and reseat the hardware first, because those cover the majority of cases. Clear the CMOS if those steps do not resolve it, or if the problem started right after a BIOS or hardware change.
Need a hand?
If you would rather have someone walk through it with you, our team does this all day. Reach them at NZXT Support.
