"The computer won't turn on" actually describes very different situations. A completely dead PC, one that spins up its fans but shows no picture and one that shuts down after a few seconds have different causes and need different checks. The method I use rests on a simple principle: start from the most trivial and likely causes, change one thing at a time and observe carefully what happens.
First: describe the symptom
Before touching anything, it's worth noting exactly what happens when you press the button:
- No reaction at all: no fans, no lights.
- Lights on the motherboard but no reaction to the button.
- Fans spinning but no picture on the monitor.
- Powering on and off after a few seconds, perhaps in a loop.
- Picture present but the operating system doesn't start.
The last case is almost always a drive or operating system problem, not a power-on one; here we focus on the first four.
Case 1: no reaction
Starting with the obvious isn't trivial: these causes are obvious precisely because they get overlooked.
- Do the socket and power strip work? Try another socket or another appliance.
- Is the switch on the back of the power supply in the on position?
- Is the power cable firmly plugged in at both ends?
- Is the case button connected to the right motherboard pins?
To rule out the button, you can briefly short the two power pins on the front-panel header with a screwdriver, having identified them in the manual. If the PC starts, the problem is the button or its cable.
If still nothing happens, the main suspect becomes the power supply. The most reliable way to check is to try a power supply that is known to work.
Case 2: lights on but no reaction
The motherboard is getting standby power, so the socket and power supply work at least in part. Check:
- that the 24-pin connector and the processor's 8-pin connector are fully seated;
- the power button connection;
- possible short circuits: a misplaced standoff or a screw that has fallen behind the motherboard can prevent booting.
Case 3: fans on, no picture
This is the most frequent case. Typical causes, in order of likelihood:
- Monitor connected to the wrong output: with a dedicated graphics card, the cable goes into the card, not the motherboard outputs.
- Monitor on the wrong input or a faulty cable.
- Memory not properly seated or in the wrong slots: remove the modules and reinsert them firmly.
- Memory training on first boot or after a settings change: on some platforms it takes minutes of black screen.
- Unstable BIOS settings, for example an overly aggressive memory profile.
- BIOS not compatible with the newly installed processor.
For point 5 you can reset the BIOS to defaults: with the dedicated button if there is one, with the jumper shown in the manual, or by removing the coin-cell battery for a few minutes with the PC unplugged.
Diagnostic LEDs and codes
Many motherboards have four small LEDs labelled CPU, DRAM, VGA and BOOT: the one that stays lit shows where booting stops. Others have a display with a hexadecimal code or a small speaker that emits beep sequences. The meaning is always in the manual, and it saves a lot of time.
Case 4: turns on and off
A power-on/power-off loop often means a protection is kicking in:
- a short circuit between motherboard and case;
- a missing 8-pin processor connector;
- a cooler not fitted correctly, with the processor overheating within seconds;
- incompatible or unrecognised memory, on some platforms.
The minimal configuration
When the previous checks aren't enough, move to a minimal configuration: motherboard, processor with cooler, a single memory module, power supply and, if the processor has no integrated graphics, the graphics card. Everything else is disconnected: drives, add-in cards, USB devices, extra fans.
If possible, do the test with the motherboard outside the case, resting on its box, to rule out short circuits. If the PC boots in minimal configuration, add components back one at a time, trying again each time. The component that brings the problem back is the culprit.
With memory you can also try each slot and each module on its own: it's a simple way to identify a faulty module or slot.
What not to do
- Don't take the power supply apart: it contains capacitors that can stay charged.
- Don't change several components at once: you lose the ability to tell what fixed it.
- Don't force connectors or processors: if something won't go in, it's probably oriented wrongly.
In my work, almost half of the "dead" computers I see come back to life within the first two steps: a cable, a connector, a reseated memory module.
In short
- Describe the symptom precisely before intervening.
- Start with socket, switch, cables and button: they're the most overlooked causes.
- With no picture, check video output, memory and BIOS settings.
- Use the motherboard's diagnostic LEDs and codes.
- With a minimal configuration you isolate the faulty component, one at a time.
