A multimeter turns SMPS testing from opinions into numbers. The trick is knowing which wire to touch, what the reading should be, and the one condition — load — without which even a perfect reading can lie.
Of all the home tests, this is the one that produces evidence. The ATX standard gives every rail an allowed band — ±5% on the main rails. Inside the band, the supply is doing its job. Outside it, you are no longer guessing: the unit is out of specification. This page gives you the wire colours, the exact numbers, and the honest warning that separates a real measurement from a comforting one.
Set the multimeter to DC volts, on a range above 12 V (or auto-ranging). The black probe is ground — it goes into any black wire pin of the 24-pin connector, and stays there. The red probe touches the coloured wire of the rail you are reading: yellow for +12 V, red for +5 V, orange for +3.3 V.
Probe from the back of the connector while it is plugged into the motherboard, so the system stays connected and alive. Everything is measured on plugs and wires — the meter never goes inside the sealed unit.
The tolerance band is the whole judgement. A rail at 12.1 V is healthy. A rail at 11.5 V is inside the band but low — legal, worth watching. A rail at 11.2 V is out of specification: the supply has failed its standard, whatever the fan is doing.
+12 V feeds the CPU and graphics card — the rail that matters most in modern PCs.+5 V and +3.3 V feed drives, USB and board logic.+5 VSB (purple wire) is live whenever the wall switch is on — it powers the start circuit itself.Here is the part most quick guides skip. A tired supply often reads beautiful numbers with nothing connected — the paperclip arrangement. Weak capacitors and tired regulation hide at zero load and collapse only when the processor and card start pulling current.
So measure in the state that fails. If the PC dies in games, the reading that matters is the +12 V rail during the game. A rail that holds 12.0 V at the desktop and slides to 11.3 V under load has confessed — in numbers nobody can argue with.
A healthy rail is boring: it sits on its number and barely breathes. A dying rail wanders — flickering tenths of a volt, dipping when a drive spins up or a fan kicks in. That instability is ripple and sag arriving before total failure.
If readings wander and the PC has the matching symptoms — random shutdowns, no-display starts, drives dropping — you do not need a laboratory oscilloscope to act. The pattern plus out-of-band dips is enough to take to the swap test.
Honesty cuts both ways: a multimeter reads average voltage, not the fast noise (ripple) riding on it, and it cannot watch the millisecond sag that resets a board. A supply can pass the meter and still be the guilty part — rarely, but truly.
That is why the order in this series ends where it ends. Meter readings outside tolerance convict the SMPS on their own. Readings inside tolerance with matching symptoms still go to the swap test — the one test that misses nothing.
Watch the video for the complete SMPS checking routine, then use this tolerance table to judge your own readings.
▶️ Watch Full Video11.5 V is inside the ATX band (11.40 to 12.60), so the unit is within specification — but it is running near the floor. If the PC is symptom-free, note it and move on. If you have shutdown or no-display symptoms, watch that rail under full load: if it dips below 11.40 V when the PC works hard, the supply has failed the standard and should be replaced.
BIOS and software readings come from the motherboard's own sensors, which are often inaccurate by several tenths of a volt — fine for a rough hint, useless for a verdict near the tolerance edge. A basic multimeter costs little and reads the connector directly. For a decision about replacing the supply, trust the meter.
Perfect steady-state numbers do not clear the supply completely — fast ripple and millisecond sags are invisible to a meter. This is exactly the case the swap test exists for: run the failing workload on a borrowed known-good unit. A clean run convicts your supply despite its good manners on the meter.