Technicians use an oscilloscope to measure . Even if voltage reads 12.0V, electrical noise (ripple) on the line can corrupt data. The ATX standard dictates that 12V ripple must be under 120mV. High ripple causes BSODs and instability that standard multimeters cannot detect.
Find the 24-pin motherboard connector. Locate the Green wire (PS_ON) and any Black wire (Ground). how to check psu