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Check for a sticking button when a PC mysteriously powers up after shutdown

Don't blame mysterious powerups on artificial intelligence inside your computer. As one TechRepublic member discovered, simple physical glitches can usually be held responsible.

TechRepublic member MiracleMax has a bewildering problem, and although it's tempting to blame mischievous computer gremlins or the angered ghosts of former support techs, there must be a rational explanation. After shutting down his NEC desktop, it will mysteriously reboot on its own a few seconds later. This is an AMD system with a 450-MHz K-6 processor, 256 MB of RAM, and a 30-GB Maxtor hard drive. It's running Windows 98SE as the operating system.

Max has checked and disabled all power settings for the BIOS, the NIC card, and in the Windows Control Panel. In addition, he has disabled the Windows fast shutdown feature through a registry edit (see Microsoft Knowledge Base article Q202633). And according to his up-to-date version of AVG 6.0, the machine is virus free.

What should Max do?
System administrator Blaine Moore is quick to suggest a BIOS update. And although flashing the CMOS with the latest version of the BIOS can often help solve problems such as this, MiracleMax rejects the suggestion. Max is probably already running the latest version of his machine's BIOS.

TheChas offers some interesting and helpful advice. To determine whether the problem is hardware or software related, he suggests booting from a floppy and then powering down in DOS (usually accomplished by continuously depressing the power button for about five seconds). If the machine reboots after this shutdown, Windows is not to blame. If hardware is the likely culprit, check the front panel on/off switch with an ohmmeter. "Make sure…the contacts open and close."

Other possible hardware causes might be a faulty power supply. If a new power supply remedies the situation, a bad 5-volt turn-on circuit on the old power supply could be to blame. If the problem persists after installing a new power supply, the power control circuit on the motherboard could have a defect. In TheChas' experience, "Over time, dust builds up on the wires from the On switch. When someone hits the button and is static charged, [it is] discharged through the button into the 'turn-on' circuit. Eventually, the circuit gets enough damage that it keeps the PC on all the time."

If this is the case, MiracleMax should replace the motherboard. Another way to work around this problem would be to install an on/off switch in the AC power line and shut off the electricity immediately following shutdown.

DR The Corporate Groups concurs with TheChas' advice about checking for a bad power switch. "My on/off switch sticks, and I have to make sure it isn't sticking or my computer will automatically come on again after I shut it down."

This sounds exactly like what MiracleMax is experiencing. A "sticky" button or burnt circuit is most likely to blame and can probably be repaired thanks to the advice of TheChas and DR. Don't overlook a simple physical defect the next time you encounter a baffling glitch. Often, cheap parts, poor construction, or spilled soft drinks and grande cafe lattes are to blame.
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