Modern motherboards require two separate power connections from the power supply: the 24-pin ATX connector for general motherboard power and a 4-pin or 8-pin CPU (EPS) connector dedicated to powering the processor. According to CompTIA Core 1 (220-1201) power supply installation and troubleshooting guidelines, failing to connect the CPU power connector commonly results in a system that does not boot and may display a critical or CPU-related error.

In this scenario, the user connected power to the motherboard, graphics card, and storage devices, but the system fails to start. This strongly indicates that the CPU power connector was overlooked, which is a frequent mistake during PSU replacement. Without this connection, the processor cannot initialize, preventing POST.

Insufficient wattage typically causes instability, shutdowns, or reboots under load rather than immediate failure with a critical error. RAM upgrades are unrelated to PSU replacement, and redundant power supplies are used in enterprise systems, not standard gaming PCs.

CompTIA emphasizes verifying all required PSU connectors, especially the CPU power connector, as a critical troubleshooting step when a system will not power on after a PSU replacement.