I am working on a clients computer and his computer turns on runs fine then after 15 min shuts down. It does not have a overheating problem. I've cleaned it out and re-applied thermal compound. I've tested the battery and works fine. computer powers on by plug and without the battery but after 15 min still shuts down. I've configured the power management settings to default. Ive done stress test on CPU and RAM, Error checks on motherboard and Hard drive and also replaced the CMOS battery. Ive checked the voltage meter on the charger and also checked the power jack. Everything seems to run fine and all error checks comes out ok except the fact that the laptop keeps shutting down. I'm stumped. I was thinking the CPU but it registers fine through the stress test. the other alternative could be the motherboard itself. Circuitry is not flowing through like it suppose to. that would be my guess.Im no expert on diagnosing a motherboard but so far all leads point to a bad motherboard. The Notebook is a STI infinity IS1422. Please if anyone has any ideas... Thanks

Assuming that all of your hardware is OK, are you sure there isnt a task scheduled to shut the system down after 15 min? maybe malware that is doing it?

To rule out software, i'd boot the system to a bootable linux distro and let it run and see if it shuts down after 15 min. if it does, the problem would definately lead to something hardware related.

commented: Good suggestion Jorge! +13

Did you check the BIOS setting. Try to reset the Bios setting. It it does not work then de as per @AleMonteiro

One last question. Does it shut down after 15 minutes of user inactivity? Does it keep running if you keep "diddling" the system and running programs? If either is the case, then it has to be some obscure power configuration setting, at least that's what it would seem to me. If not, then I would agree with JorgeM that it may be a hardware issue.