The fans spin. The lights glow. The screen stays black and the keyboard lights never wake. It looks alive and dead at the same time — and one of the most common reasons is a power supply that can start, but cannot deliver.
This fault confuses everyone because the PC looks half alive. Fans and LEDs run on simple power — they ask little and forgive a lot. The processor and motherboard are not forgiving: if the +12 V rail sags or the power-good signal never arrives cleanly, the board refuses to start its real work while the fans spin on, lying to you. Here is how to read this pattern and prove which part is guilty.
A fan is a simple motor. Give it roughly the right voltage and it turns, a bit slower or faster, without complaint. A processor is the opposite: the motherboard watches the supply rails and the PWR_OK signal, and only releases the CPU to start when the power is stable and inside tolerance.
That gap is the whole mystery. A tired SMPS — aged capacitors, a sagging +12 V rail — clears the fan bar easily and fails the processor bar every time. Result: a cabinet full of motion and light, and a board that never truly wakes.
Before blaming power, clear the two-minute checks — this symptom has innocent causes too:
If none of these change anything and the keyboard lights never flash on power-up, the board is not starting — and power quality moves to the top of the list.
Weak-supply no-display has a smell of its own. It often appears after an upgrade — a new graphics card asking more than the old unit can give. It may start sometimes, especially when cold in the morning, then refuse after a warm restart. HDD clicks, USB devices dropping, or the PC needing two or three presses of the button all travel with it.
And the classic: the same PC starts fine in the repair shop. Different workload, different moment, a borderline rail behaves. Borderline is the key word — this fault lives on the border.
A multimeter turns the guess into evidence. Measure at a spare connector while the PC is connected and attempting to run:
+12 V must hold between 11.40 V and 12.60 V — watch it during start-up, not just at rest.+5 V between 4.75 V and 5.25 V; +3.3 V between 3.14 V and 3.47 V.A rail that dips, wobbles or arrives late while the fans spin has told you the story. Numbers outside the band are a verdict, not an opinion.
Borderline faults deserve the decisive test. Connect a known-good SMPS of equal or higher wattage, leave everything else untouched, and power on.
Never keep running a borderline unit while you decide. A sagging rail stresses everything it feeds.
See the video for the live testing method that separates a weak SMPS from a motherboard fault in this exact symptom.
▶️ Watch Full VideoIt is one of the classic weak-supply patterns. Heat raises resistance and weakens tired capacitors further, so a borderline rail that just holds when cold dips below tolerance when warm. Measure the rails during the failing warm start, and confirm with a swap test.
Yes — a dead graphics card gives a black screen too, but usually the keyboard lights flash and the board clearly starts (you may even hear the Windows sound). If your board has integrated graphics, test with the card removed and the cable on the motherboard port. No keyboard activity at all points earlier — at power or the board.
Either is possible, and the fix differs. Add up the real draw: if the card pushed the system past what the unit can deliver, you need a bigger unit. If the wattage was always enough but the rails sag, the unit is dying and a same-size quality replacement is the answer. The multimeter readings tell you which story is yours.