Testing a PSU Before You Replace Anything Else

When a machine stops posting, the cheapest component to rule out is the power supply, and a basic tester does it in about a minute. It is worth owning for one reason and worth distrusting for another: it reliably identifies a dead supply, and it cannot identify a weak one.

A note on this guide: this is written from the ATX specification and the documented behaviour of these testers, not from bench work here. See how we test.

The problem, as it presents

A machine that worked yesterday does nothing today, or spins fans for a second and stops, or posts intermittently. The symptom points nowhere in particular, and the components it could be — board, supply, memory, card — are ordered from most to least expensive in almost exactly the wrong order for guessing.

So people guess anyway, usually starting with the motherboard because that is what the internet says. It is the most expensive and most laborious swap available, and it is frequently not the fault.

Why swapping parts first disappoints

Substitution is a real diagnostic method, but it requires spare parts, and the spare almost nobody has is a second power supply. So the sequence becomes: buy a board, fit it, discover nothing changed, and now own a board and still have the original problem.

The power supply is worth eliminating first not because it is the most likely culprit but because it is the cheapest to check. Whether it is likely and whether it is cheap to rule out are different questions, and only the second one should decide the order.

What actually matters

That it reads the rails, not just the fan. The free version of this test — bridging the green wire to ground with a paperclip — proves the supply switches on. That is all. A tester with a display shows the 12V, 5V and 3.3V rails and tells you whether each is inside the plus-or-minus five per cent the ATX specification allows.

That it reports power-good. This is the signal the supply sends the board to say its rails have stabilised. When its timing drifts out of specification, the machine will not start even though every voltage reads correctly. It is exactly the fault that looks like a dead motherboard, and a tester that shows the power-good figure catches it.

Understanding that it tests unloaded. A tester draws a negligible fraction of a supply's rated output. A unit whose capacitors have aged badly can hold correct voltages with nothing attached and collapse the moment a graphics card asks for real current. This is the tool's honest limit, and it is why a pass is weaker information than a fail.

Whether the wall socket is the actual problem. Occasionally the supply is fine and the outlet is not — no ground, a shared circuit sagging under load, or a failing extension lead. An outlet tester answers that in seconds and costs very little.

The options, and who each suits

A basic ATX tester with a display suits anyone who has built or repaired more than one machine. It is small, it needs no power beyond the supply under test, and it turns an open-ended question into a one-minute check.

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An outlet tester suits anyone whose problems are intermittent, or whose lights dim when something else in the house starts. It rules out the room rather than the machine, which is a category of fault people almost never check.

A multimeter does everything a tester does and more, if you already own one and are comfortable back-probing a live connector. This site already covers choosing a multimeter from the automotive side, and the same instrument works here.

Substitution with a known-good supply remains the only conclusive test, and it is what a repair shop does. If a spare is available, use it and skip everything above.

The mistake to avoid

Treating a pass as an all-clear and moving on to expensive parts. The correct reading of a passing test is "the supply is not dead, and it may still be weak" — particularly on a machine that fails only under load, which is the classic signature of a supply that cannot hold its rails when a graphics card draws hard.

If the symptom is only under load, a passing bench test has told you very little, and the next step is substitution rather than a shopping trip. Where power problems sit among the other things that go wrong is covered in what goes wrong with a gaming PC after year one.