Dell computers have an interesting self-diagnostic capability that can be helpful. If you don’t have any computer diagnostic tools, there is a free tool from Dell to be used. The tool can help, but not in all cases. It is usually located at the back of the PC where all peripheral connections are.
Legend: = off = on * = flashing Y = Yellow G = Green
Below is a displayed diagnostic code that is a motherboard fault. This is usually caused by a bad connection. When you have this problem, try to disconnect all the cables and all the components and power it on.
Figure 1: Motherboard issue
Here is a LED code that tells you that something is wrong with the memory. The best way to fix this is to use good memory modules.
Figure 2: Memory issue.
If something is wrong with the VGA card or with video that is integrated on the motherboard, this will be the displayed codes below.
Figure 3: Video / PCI card issue.
Figure 4: Disk drive / external device issue.
Traditional diagnostic tools like UBCD4WIN and Memtest will work on Dell desktops.
What most likely can fail on Dell desktops is the hard drive. The hard drive takes a heavy usage load, and that's why they tend to fail more than compared to other elements. If we know that most likely this would be the hard drive fault, we would test it with bootable UBSD4WIN, or by taking it out and testing with HDTune.
Sometimes the hard drive fails because of a faulty power supply unit. The capacitors in power supply units dry out and the voltages that reach the hard drive get voltage fluctuations.
The second fault most likely is the virus infection. The best way to diagnose this is to run a bootable CD like Avira Rescue Disk. There is a high chance to infect another system if we take the hard drive and connect it as external hard disk. The best way to cure a virus infection is to reinstall the operating system. If the infection is not severe, the best thing to do is then run MalwareBytes in Safe Mode and remove it.
If we go further on diagnosing computer problems on Dell desktops, then the next common fault is the failing motherboard. The elements that get a very heavy load on the motherboard are power elements like capacitors and MOSFETs. The capacitor problem is widely known. When a capacitor dries out on the motherboard, then the pc does not post or boot and it can't install the operating system. Sometimes this happens in the evening when everything is working perfectly and then it will not boot in the morning. When the capacitors are faulty in the CPU voltage converter, usually the pc does not even post. When the capacitors are faulty in the other parts on the MB, usually the system is unstable and works in the evening and does not work in the morning.
Figure 5: Faulty capacitors on the motherboard.
Here is shown how to test the 12v rail that goes to the CPU. On the motherboard there is auxiliary connector and the plug usually has two yellow wires and two black wires.
Figure 6: Multimeter probes connected to the CPU +12v socket.
Set the multimeter to measure resistance and connect the probes as it is shown above. One probe goes to the ground pin and another probe goes to the +12v pin.
Figure 7: The multimeter is set to measure a resistance
If readings are small numbers like 20 to 2, then the motherboard is a faulty MOSFET and it is not recommended to power it on because it can damage the power supply unit. The MOSFET or the motherboard must be replaced. If the readings show about 100 at the beginning and then increases to infinity in a few seconds, everything is okay. The changing readings here are because the capacitors charged up from the multimeter.
Figure 8: Resistance readings from CPU 12v socket. This indicates a faulty MOSFET.
The next fault would be a RAM fault. This is very rare but happens when it is not expected. Usually the system doesn’t show any specific symptoms, but the OS installation can fail or can exhibit other behaviours categorized as a rock enrol.
Diagnose the memory with Memtest. The memory that is available on the market is dynamic and is very sensitive to voltage fluctuations. Higher voltage can damage the RAM modules.
The next component to look for is the CPU. As this is very rare, it does happen and did in my practice a few times when the CPU was faulty. The symptoms are from strange behaviours that happen in the system like not starting at all or showing a blue screen.
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