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Difficulty: EasyMotherboard, RAM, CPU, and Power Issues

A technician replaces a faulty power supply in a desktop workstation. After connecting the 24-pin main ATX power connector to the motherboard and powering on the system, the case LEDs light up briefly, but the computer does not POST and the CPU fan fails to spin. Which of the following is the most likely reason for this failure?

  1. The 4-pin/8-pin EPS12V CPU power cable was not connected to the motherboard.Answer
  2. B
    The memory modules were installed into alternating dual-channel memory slots.
  3. C
    The BIOS/UEFI beep code speaker is plugged in with reversed polarity.
  4. D
    The CMOS battery has lost its charge and needs to be replaced.

Answer

The 4-pin/8-pin EPS12V CPU power cable was not connected to the motherboard.
Modern CPUs require dedicated power provided by an auxiliary 4-pin or 8-pin ATX12V/EPS12V connector. Connecting only the 24-pin main power connector provides standby power to motherboard LEDs, but leaves the CPU completely unpowered, preventing the CPU fan from spinning and preventing POST.

Step-by-Step Solution

1
Identify system power requirements
Recognize that desktop motherboards require both the main 24-pin ATX power connector and an auxiliary 4/8-pin CPU power connector.
The processor draws power from dedicated motherboard VRMs powered by the auxiliary EPS12V/ATX12V connector.
2
Analyze reported symptoms
Main 24-pin power delivers standby voltage to case LEDs, but the processor receives no power.
Without auxiliary CPU power, the CPU remains unpowered, halting POST and keeping CPU fan headers disabled.
3
Determine corrective action
Plug the auxiliary 4-pin or 8-pin power cable from the power supply into the CPU power header on the motherboard.
Connecting all required power leads restores complete power delivery to the system.

Key Concept

CPU Auxiliary Power Connection
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Topics

Motherboard, RAM, CPU, and Power Issues

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