PC dead, or switching off in the middle of work? Half the time the finger points at the SMPS — and half of that time the SMPS is innocent. This is the full testing order, from the wall socket to the final verdict, so you only replace the part that is actually guilty.
A dead PC creates one expensive habit: buying parts on a guess. The SMPS (the power supply unit) is the most blamed and the least tested part in the box. This guide runs the same order a technician uses — free checks first, simple tests next, and only one test at the end that gives the honest final answer. Work through it in order. Do not skip to the buying step.
More dead PCs are revived here than anywhere else. Before any tool comes out, prove that power is actually reaching the box.
If the PC starts on any of these, your SMPS was never the problem.
With the SMPS disconnected from the motherboard, a paperclip joining the green PS_ON wire to any black ground wire on the 24-pin connector tells the unit to start. If the fan spins, the unit is alive enough to start.
Be honest about what this proves: starting is all it proves. It says nothing about whether the voltages stay correct when the PC actually pulls power. A unit can pass this test proudly and still kill your PC under load — that case has its own page in this series.
A multimeter on the connectors tells you what the motherboard is really being fed. ATX tolerance is ±5% on the main rails:
+12 V rail must read between 11.40 V and 12.60 V+5 V rail must read between 4.75 V and 5.25 V+3.3 V rail must read between 3.14 V and 3.47 VThe honest catch: readings taken with nothing connected can look perfect on a dying unit. Measure with the PC connected and running if you can — a rail that sags only under load is exactly how weak supplies fool every quick test.
Some evidence needs no instrument at all. A burning or chemical smell, scorch marks near the fan grill, a top cover bulging over swollen capacitors, or loud buzzing and clicking from inside the unit — each one ends the investigation on the spot.
Do not keep testing a unit like this, and do not keep using it while you think it over. A supply that smells burnt is not a diagnosis puzzle; it is a replacement order. Unplug it and move to the last step.
Every test above narrows suspicion. Only one test settles it: connect a known-good SMPS of equal or higher wattage and run the PC through the exact work that was failing.
Borrow before you buy whenever you can. It is the cheapest certainty in PC repair.
The complete SMPS check — every test in this guide, performed live on a real unit — is in the video above.
▶️ Watch Full VideoNo — and that is the point of the test order. Dead sockets, dead cables, a knocked-off rear switch and a failed spike strip all imitate a dead SMPS perfectly. Run the free checks first, then the paperclip test, and let the swap test give the final word before you buy anything.
Never. The capacitors inside hold a dangerous charge long after the plug is pulled, and a sealed SMPS is treated as a replace-only unit even by most technicians. Every safe test is done from outside — connectors, fan and meter probes. If the unit is faulty, replace it.
It means the SMPS can start — nothing more. The paperclip test puts almost no load on the unit, so weak rails that collapse under a real load still pass it. Confirm with multimeter readings under load, and settle the question with the swap test.