Its quite hard sometimes to troubleshoot a computer that is not booting up properly especially when it shuts down a few seconds after powering on. For other problems, you can go by them easily by just opening some types of programs like Dr Watson (in case you are a Windows addict) or examining error messages that pops up after booting. But the issue we have at hand is quiet hard, you never get a chance to see the start button or mouse cursor. All you see are a few POST (power on self test) lines and then the system shuts down!
A computer may shutdown because its processor maximum operating temperature has been exceeded, one or all of its RAM (random access memory) has failed or the BIOS has told it to shutdown. This kind problem requires ‘logical elimination’ if you don’t mind me putting it that way. You would have to verify that something is working until you single out what is not working properly ( or not working at all) in the system. Lets use the ‘logical elimination’ on the problem at hand. Firstly, check the processors heat sink for any abnormal temperature rise. Overheating processors may cause the computer to shutdown or fly it a few seconds after powering on. In case of some laptops you will have to remove the keyboard to access the heat sink or in some models you will have to remove a cover on the bottom side of the laptop. Switch on the laptop (if possible) with base cover or keyboard removed, and examine any signs of overheating. Feel the heat sink with your hand, remove it after a few seconds to avoid burns. Some overheating heat sink may produce a smell, look out for that too.
Desktop computers are quiet easy to access the heat sink. Some have a side cover, you just need to unscrew it. DELL desktops may have non-standard cases which are difficult to open for new technicians, you may have to look out for buttons somewhere on the case press and hold them while opening otherwise you will spend hours trying to open the case. After opening switch power on and examine the heat sink. Its located above microprocessor on the motherboard. So if you see that there is excessive heat, it might be that the system fan is dead or there is none, check. make sure there is one and its working properly. If its not working properly, replace it. Remove any dust and foreign particles between the heat sink and fan. Dust and foreign particles acts as heat insulators.
You have reached this far but system still shuts down, then the problem might be one or all of your RAM. Remove them and replace with a known good one. When ever you are removing or replacing RAM, never touch its contacts!
Changing RAM did not help? well try to obtain and upgrade your computer BIOS or flush it if necessary. Some (but not all) corrupted BIOS may cause this kind of problems. Make sure the BIOS chip can be upgraded. If the chip is plugged in a socket then its upgradable, if its soldered to the motherboard then leave it as it is and try to flush it. If there is power failure during BIOS flushing, your motherboard will not work again. So always have a backup power like UPS when performing this task. Sometimes a bad processor may cause this problem, try also to change the processor with a known working one.
If all of the above does not work for you, its time to replace the motherboard.
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