PC Turns On Then Off Immediately? 7 Causes, Quickest First

In this guide (9 sections)

• Why a PC switches itself off
• 1. Something is shorting against the case
• 2. The wrong cable is in the CPU socket
• 3. A power cable isn’t fully seated
• 4. The RAM isn’t seated or isn’t working
• 5. The processor or its socket has a problem
• 6. The cooler isn’t doing its job
• 7. The power supply, button or motherboard is failing
• Real cases: what other users ran into

You press the power button, the fans spin for a moment, and then the PC switches itself off. Sometimes it restarts and does it again, in a loop. It’s alarming, but there’s a reassuring explanation: in most cases the power supplyPSU (power supply): The power supply: the box that takes power from the wall and feeds every part through cables. More in our PC basics glossary →Close ✕ is shutting down on purpose, because it detected something wrong. Find that something and the PC usually starts normally.

Why a PC switches itself off

A good power supply constantly watches for danger: a short circuitShort circuit: Electricity taking a path it shouldn't, for example through the case. The PC then shuts off to protect itself. More in our Startup & BIOS glossary →Close ✕, too much current, or a part that isn’t getting power properly. If it spots one, it cuts the power in a fraction of a second. That’s short-circuit protectionShort-circuit protection (OCP/SCP): Safety features in the power supply that cut the power instantly if something is wired wrong or shorting. More in our Power supplies & cables glossary →Close ✕, and it’s the reason a wiring mistake usually ends in an instant shutdown rather than burned parts.

How long the PC stays on is a useful clue:

How long it stays on	Most likely cause	Where to look
It stays on for a split second (the fans twitch).	A short circuit or a wrong cable	Causes 1, 2 and 3
It stays on for 2–5 seconds, then turns off, sometimes repeating.	Missing CPU power, RAMRAM (memory): Memory modules: short-term working space the PC needs to run. They clip into slots next to the processor. More in our PC basics glossary →Close ✕ problem, or the motherboard’s own safety check failing	Causes 2, 4 and 5
It stays on for 15–30 seconds, then turns off.	The processor overheating with no cooling	Cause 6
It only stays on while you hold the button.	Power button, power supply or motherboard fault	Cause 7

1. Something is shorting against the case

The motherboard must only touch the case through its standoffs, the small metal spacers under its screw holes. If it touches the metal anywhere else, through a loose screw, an extra standoff, or the motherboard screwed straight onto the case, the power supply sees a short and shuts down.

How to check: take the motherboard out and run it on its cardboard box with just the processor, cooler, one module of RAM and the power supply connected. If it stays on outside the case, the problem is inside the case. Our minimal boot test shows exactly what to keep connected.

2. The wrong cable is in the CPU socket

This one is surprisingly common. The processor’s 8-pin power cable and the graphics card’s 8-pin cable look almost identical, and with some force one can be pushed into the other’s socket. They’re wired differently, so using the wrong one makes the power supply shut down immediately.

CPU 8-pin vs graphics card 8-pin: not the same cable

They look alike, but they're wired differently. Check the label printed on the plug.

CPU (EPS) 4+4

Label on the plug: "CPU"

Two halves that clip together.

Goes into:

the motherboard, top-left corner, near the processor.

Common mistake:

plugging in only one half. Both halves must go in.

Graphics card (PCIe) 6+2

Label on the plug: "PCIe" or "VGA"

A 6-pin part plus a small 2-pin part.

Goes into:

the graphics card itself, on its top or end.

Common mistake:

forcing it into the CPU socket. The PC then shuts off instantly.

Never force a plug. The right one slides in and clicks with gentle pressure. Pin shapes shown are simplified.

Fix: check the label printed on the plug in the top-left socket. It must say CPU. If it’s a 4+4 connector that splits in two, make sure both halves are in.

3. A power cable isn’t fully seated

A half-inserted 24-pin cable, or only one half of the CPU cable, can let the PC start for a moment and then drop out. On a modularModular power supply: A power supply whose cables detach. Each cable must be plugged in at both ends: the PSU and the part. More in our Power supplies & cables glossary →Close ✕ power supply, also check the other end of each cable, where it plugs into the power supply. Our guide for newly built PCs covers both connectors in detail.

4. The RAM isn’t seated or isn’t working

A RAM module that isn’t fully clicked in, or that the motherboard can’t run, can make some PCs start, fail their startup check and power off, then try again in a loop. This is especially common right after fitting new RAM or enabling XMPXMP / EXPO: A one-click BIOS setting that runs RAM at its advertised faster speed. If unstable, the PC may not start. More in our RAM & storage glossary →Close ✕ or EXPO.

Fix: reseat the RAM until both clips click, try a single module, and if the problem started after a BIOSBIOS (UEFI): The motherboard's built-in settings program, which starts the PC before Windows. You open it with Delete or F2 at startup. More in our Startup & BIOS glossary →Close ✕ change, clear the CMOS. Our no-display guide shows how to reseat RAM properly.

5. The processor or its socket has a problem

Bent pins in the processor socket (Intel) or on the processor itself (older AMD) can cause a start-and-stop loop. So can a processor the BIOS doesn’t support yet.

How to check: with a flashlight, look at the socket at an angle: the pins should form an even, regular pattern. If you’ve just fitted a new processor, check the motherboard’s CPU support list to see whether it needs a BIOS updateBIOS update: A new version of the motherboard's built-in software, often needed to support newer processors. More in our Startup & BIOS glossary →Close ✕ first.

6. The cooler isn’t doing its job

If the PC runs for 15–30 seconds before cutting out, the processor may be overheating and shutting itself down. On a newly built PC, the usual reasons are a cooler that isn’t sitting flat, the plastic film still on its base, or the cooler’s fan not plugged in.

Fix: check the cooler is screwed down evenly, the protective film is gone and there’s thermal pasteThermal paste: A thin gray paste between a chip and its cooler that helps heat pass from one to the other. More in our Cooling & noise glossary →Close ✕ between the cooler and the chip. Make sure the fan or pump is connected; our CPU fan error guide shows which connector to use. If the PC stays on but the processor runs hot, check what CPU temperature is safe while gaming.

7. The power supply, button or motherboard is failing

If none of the above applies, and especially if the PC only stays on when you hold the power button, a faulty part becomes likely.

• Power supply: the easiest part to test by swapping in a known-good one. A power supply damaged in shipping can behave like this. Our guide on signs your PSU is failing shows other warning signs and safe tests.
• Power button: bypass it by briefly touching the two power-switch pins on the front-panel connectorFront-panel connector: A small group of pins on the motherboard where the case's power button, reset button and lights connect. More in our Startup & BIOS glossary →Close ✕ with a screwdriver. If the problem stays the same, the button isn’t the cause.
• Motherboard: if a different power supply and the minimal boot testMinimal boot test: Starting the PC with only the essential parts connected, to find which part is causing the problem. More in our Startup & BIOS glossary →Close ✕ don’t help, the motherboard is the next suspect. If it’s new, contact the seller.

Real cases: what other users ran into

These are real reports from PC forums, summarized in our own words, with a link to each original thread.

Real caseASRock H370 Fatality · Core i5-8400 · Corsair CX750MSolvedA graphics card cable in the CPU socketRead what happened

A newly built PC kept powering on and off in a loop. The owner had checked the socket pinsCPU socket and pins: The square slot where the processor sits. Its tiny pins bend easily, which can stop the PC from starting. More in our Startup & BIOS glossary →Close ✕, reseated the processor and RAM, tried different RAM slots and tested the power supply in an older PC. A moderator pointed out that the plug in the CPU socket was a 6+2 graphics card plug, which isn’t electrically compatible with the 4+4 CPU connector. The owner had worked it out that same morning, and the PC was up and running.

Takeaway: check the label on the plug in the CPU socket. “PCIe” or “VGA” means wrong cable. See cause 2.

Source: ASRock Forum

Real caseRyzen 5 2600 · ASRock B450 Pro4 · RX 570Solution not confirmed by the userAn on-off loop caused by bent processor pinsRead what happened

The PC ran for about 15 seconds, switched off, restarted and repeated until the power supply was switched off. The owner checked the RAM slots, cleared the CMOS and tested the motherboard outside the case, where they found a RAM module that wasn’t fully seated. After more testing, they found bent pins on the processor and arranged a return. They didn’t post an update after the new processor arrived.

Takeaway: if cables and RAM check out, look closely at the processor pins or socket. See cause 5.

Source: ASRock Forum

Real caseRyzen 7 5800X3D · ASUS ROG B550-F · RX 7900 XTX · Corsair RM850x SHIFTNot solved by the userOnly stays on if the button is held downRead what happened

After moving house, the PC woke up for half a second and died every time the button was pressed, unless the owner held the button for about a second. The graphics card’s power-connector warning lights also flashed. Starting the motherboard directly from its power pins did exactly the same, so the case button wasn’t the cause. The owner suspected the power supply, which had arrived damaged, but hadn’t tested a spare yet.

Takeaway: when bypassing the button changes nothing, test with another power supply before suspecting the motherboard. See cause 7.

Source: TechPowerUp Forums

Frequently asked questions

Why does my PC turn on and then off right away?

Most often the power supply is shutting down to protect the PC: because of a short circuit, a wrong or loose power cable, or a part that fails its startup check. It’s usually a wiring problem rather than a dead part.

Can a bad power supply cause this?

Yes. A faulty power supply can start and immediately drop out. Swapping in a known-good one is the most reliable way to check.

Why does my PC keep restarting in a loop before showing anything?

A start-stop loop often means the motherboard is failing its startup check, commonly because of RAM, a recent BIOS change or missing CPU power. Clearing the CMOSClear CMOS: Resetting the motherboard's settings back to factory defaults. It doesn't delete your files. More in our Startup & BIOS glossary →Close ✕ and testing one RAM module are good first steps.

Is it safe to keep trying to turn it on?

A few attempts while you troubleshoot are fine. If you smell burning or see sparks, stop immediately and don’t power it on again until you’ve found the cause.