PC powers on but shuts off after a few seconds

Fans spin, LEDs come on, and then - a few seconds later - everything cuts out. Sometimes it tries to restart itself. Sometimes it just sits there dead until you hit the power button again. This particular symptom is distinctive enough that it narrows the suspect list down considerably.

Why this happens

A PC that shuts itself off within seconds of powering on is almost always doing one of two things:

• Hitting a protection cutoff - the motherboard detects a fault condition and kills power before damage occurs
• Not receiving stable enough power to proceed past the earliest stage of startup

The difference between this and a PC that won't turn on at all is that something is working - the system starts, begins its power-on sequence, then stops. That means the fault is almost certainly in power delivery, not a completely dead component.

Start with the PSU and power connections

Check every power connector. The 24-pin motherboard connector and the CPU power connector (8-pin or 4+4-pin, near the top of the board) both need to be fully seated. The CPU power connector being loose or completely forgotten is one of the most common causes of this exact fault - the system powers on, gets far enough to try to initialise the CPU, finds it has no power, and cuts out.

Try a different wall socket and cable. Sounds basic, but a faulty extension lead or socket delivering low voltage will cause this.

Test or swap the PSU. A PSU that's failing, undersized, or simply not delivering clean power will often manage a few seconds before voltages sag below what the motherboard needs. The paperclip test (bridging green to black on the 24-pin with everything disconnected) confirms the PSU turns on, but not that it's healthy. A known-good spare is the definitive test.

Check the CPU cooler

Most modern motherboards have thermal protection that shuts the system down if the CPU temperature spikes immediately on startup - which it will if there's no cooler, if the cooler isn't making contact, or if the cooler's fan header isn't connected.

Make sure the cooler is mounted properly with even contact across the CPU lid, all four push-pin clips or screws are secure, and the fan cable is plugged into the CPU_FAN header. A cooler that's mounted on only two of four points can rock slightly and lose contact, causing an immediate thermal cutoff.

Clear the CMOS

A corrupted BIOS configuration - often after a failed update, a power cut during POST, or sometimes for no obvious reason - can cause the board to get stuck in a restart loop or cut out during early startup. Clearing the CMOS resets everything to factory defaults.

Locate the CMOS jumper on the board (labelled JBAT1, CLR_CMOS, or similar), move it to the clear position for 10-15 seconds with the machine unplugged, then return it. Alternatively remove the coin cell battery for 30 seconds. Either works.

Strip it to minimum components

Remove everything except CPU, one stick of RAM, and the PSU connections. No GPU (if your CPU has integrated graphics), no drives, no USB devices. If the system now stays on, add components back one at a time. Whatever causes the cutout when added is the problem.

Pay particular attention to the GPU when adding it back - a graphics card with a fault, or one whose power requirements exceed what the PSU can deliver, is a common cause of immediate shutdown under a system that otherwise posts.

Look for shorts

A metallic standoff in the wrong position behind the motherboard, touching solder points it shouldn't, can cause an immediate short circuit and shutdown. This usually happens after a case transfer or build, but loose standoffs can shift over time.

Remove the motherboard and inspect the back for any marks, scorch points, or standoffs that don't line up with mounting holes. Try booting the board outside the case on a non-conductive surface (the cardboard box it came in works) as a quick diagnostic.

Check for physical damage

With the system unplugged, inspect:

• Bulging capacitors near the CPU socket or in the VRM area - domed tops mean they're failing
• Burn marks around the 24-pin connector, CPU power connector, or VRM components
• Bent CPU socket pins - remove the cooler and CPU and check with a torch

Any of these point to a component-level fault that needs proper diagnosis before the machine is used again.

When to stop and send it in

If you've checked connections, cleared CMOS, stripped to minimum components, and it still shuts off within seconds, the fault is most likely a failing PSU, a VRM fault on the motherboard, or in some cases CPU damage. Separating these requires known-good spares and bench equipment to monitor what's happening to the power rails at the point of shutdown. We can catch the fault in the act rather than guessing from the symptoms - this is one of the cases where bench diagnosis saves a lot of time and avoids replacing parts that don't need replacing.