CPU LED Solid Red and Power Cycle
A technician is troubleshooting a desktop PC that fails to boot. When the power button is pressed, the system powers on for approximately 5 seconds, then shuts down and immediately restarts, repeating this cycle. The technician has replaced the power supply, reseated all components, and verified there are no loose screws or shorts. The motherboard has diagnostic LEDs, and the CPU LED glows solid red when the system attempts to start. The CPU is an Intel Core i7-12700K (LGA1700) installed in a compatible motherboard. The cooler is properly mounted with thermal paste applied. Which of the following is the MOST likely cause?
Quick Answer
The answer is that the motherboard BIOS version does not support the installed CPU. This is correct because a solid red CPU LED indicates a fault during CPU initialization, and the power cycle—where the system runs for about five seconds, shuts down, and restarts—is a classic symptom of the motherboard failing to recognize the processor. Since the technician has already replaced the power supply, reseated components, and verified the cooler is properly mounted, the issue narrows to a compatibility gap: many LGA1700 motherboards shipped without the microcode needed for Intel’s 12th-gen Alder Lake CPUs like the Core i7-12700K, so the system cannot complete POST and enters a reset loop. On the CompTIA A+ Core 1 220-1101 exam, this scenario tests your ability to differentiate between hardware failure and firmware incompatibility, a common trap where candidates overlook BIOS updates after swapping the PSU. Remember the mnemonic “Red LED, reset loop? BIOS needs a boost” to recall that a CPU LED combined with a power cycle often points to outdated firmware rather than a dead chip.
⚠ Common exam trap
Candidates often assume a CPU LED always means a physical hardware failure (e.g., bent pins or dead CPU), but CompTIA tests the knowledge that BIOS compatibility is a critical step before assuming component failure, especially with newer CPU generations on the same socket.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The motherboard BIOS version does not support the installed CPU.
The CPU LED glowing solid red indicates a CPU-related fault. Since the technician has already replaced the power supply, reseated components, and verified proper cooler mounting, the most likely cause is that the motherboard's BIOS version predates the microcode required for the Intel Core i7-12700K (Alder Lake, LGA1700). Many LGA1700 motherboards require a BIOS update to support 12th-gen CPUs; without it, the system fails to initialize the CPU, leading to the observed power cycle.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The motherboard BIOS version does not support the installed CPU.
Why this is correct
If the BIOS is outdated, it may not recognize the CPU, leading to a failure to initialize. This often causes a power cycling loop and a CPU diagnostic LED to illuminate. Updating the BIOS (possibly using a supported CPU or USB flashback) would resolve the issue.
- ✗
The CPU cooler is not making sufficient contact with the CPU.
Why it's wrong here
Insufficient cooler contact would typically cause thermal shutdown after several seconds of operation, not immediate power cycling. The fact that the system shuts down and restarts within 5 seconds suggests a different issue, such as POST failure, not overheating.
- ✗
The RAM modules are not fully seated in the correct slots.
Why it's wrong here
A solid CPU diagnostic LED indicates the processor failed initialisation, so faulty RAM seating would not illuminate that specific LED. RAM faults typically trigger memory or DRAM LEDs and POST beep codes. The LGA1700 socket likely has bent pins or an improperly seated CPU.
- ✗
The power supply unit is delivering insufficient wattage on the +12V rail.
Why it's wrong here
The technician has already replaced the power supply with a known working unit, and the system initiates power briefly. A PSU issue would more likely cause a no-power or immediate shutdown without restart, and the CPU LED would not necessarily be the primary indicator.
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Same concept, more angles
1 more way this is tested on 220-1101
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A technician builds a new PC. When powering on, all fans spin and LEDs light, but the monitor shows 'No Signal'. The motherboard has a CPU diagnostic LED that remains lit. The technician has already reseated the RAM and GPU, and verified the monitor and cable are functional. Which of the following should the technician do NEXT?
medium- A.Replace the power supply.
- ✓ B.Reseat the CPU and check for bent pins.
- C.Update the motherboard BIOS.
- D.Test the RAM in another system.
Why B: The CPU diagnostic LED remaining lit indicates a CPU-related fault. Since the technician has already reseated RAM and GPU and verified the monitor and cable, the next logical step is to reseat the CPU and inspect the CPU socket for bent pins. Bent pins can prevent proper contact between the CPU and motherboard, causing the CPU diagnostic LED to stay on and the system to fail POST.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1101 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 220-1101 exam.