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If you still haven't located the problem, then it may be a fault on the motherboard. By all means disassemble the laptop to see whether there is anything obvious, such as failing electrolytic capacitors (see the capacitors page for details), or any components which have clearly overheated, or physical damage. If the laptop powers on but powers off again within a few seconds, it could be that the heatsink is no longer making efficient thermal contact with the processor, allowing it to overheat and automatically shutdown.