Failure to Start Dimension 8400, unless I Reset the CMOS. WHY and What can I do about it?

Replace the CMOS battery (on the motherboard) and see if that helps.  Most are CR2032 batteries but check yours to verify what it is.

However, if it is a BIOS chip problem, unfortunately the only way to fix it is with a new motherboard.  The BIOS chip, only, cannot be replaced.

I don't usually suggest flashing the BIOS because of the small chance of a BIOS update failure that corrupts the BIOS.  But in your situation with an apparent BIOS problem it's worth the chance. 

HERE is the latest 8400 BIOS.  Follow the instructions EXACTLY.  If you do it from Windows, close any programs, including any antivirus programs - only have the BIOS update running.

The battery is right-side up. 

When the tower won't boot, the 4 LEDs are out.  Even though the tower is plugged in, it seems as though it isn't.  Nothing happens when I push the power button.  I've unplugged it, waited, released the "flea" power, and plugged it back in, and still nothing.  The only thing that works (so far) is opening the case, resting the CMOS, closing it, and plugging it back in and it starts up and works great.  My temporary solution is to not shut down, but go into standby mode, where shaking the mouse starts it up again....no problem.

Weird!

Leslie 

Did you install the new battery right-side up?  ;)

And what color are the 4 diagnostic LEDs on rear of the tower when it won't boot?

Ron 

Does sound like the NVRAM chip has failed as fireberd suggested.

When you shut down, do you turn off a power strip too, or is the PC connected directly to the wall?

I suppose another alternative is to leave the battery out, but then your date and clock won't be correct ever again...

Ron 

The computer is plugged into a UPS battery back up. 

Currently, I am using the computer (it's running perfectly well once it's on) and instead of shutting down, I leave it in standby mode.  I'm not sure how long that is safe to do, but I'm backing up my data and preparing for some major failure that I cannot fix.  I haven't flashed the bios....not ready to chance it, but I will try taking out the battery.  I'll let you know.

Thanks.

See what happens if you plug the PC directly into the wall instead of into the UPS... (humor me!)

Standby mode is ok, but will use more power than if it's totally off. And backing up your data is always a good idea. ;)

Ron 

Hi,

I clicked the "solution" button by accident.  Actually, so far, I have tried inserting the battery upside down, plugging the computer into a wall outlet (instead of the UPS battery), and taking the battery out all together, and I flashed the BIOS, but none of these steps has solved the problem.  I still cannot turn the computer on with the power button.

I am still leaving the computer in standby mode when not in use, and I can restart occassionally to install updates when necessary.

Do you think I may have a faulty power button after 4 years?

4 Blooms

My first reply I failed to mention that when flipping the battery over make sure all sources of power to the computer are disconnected...  You might want to try leaving it overnight unplugged and battery flipped..

looking at a lot of these forums, suddenly not powering up seems to be fairly common with Dells, not always but most have been PSU's.

I tried several battery change and power cord connection changes, but last night, flashed the BIOS, which went easily and satisfactorily.   However, there was no change in my inability to power up using the power button.  This morning, I took a closer look at the button....it is green while the computer is on, but it is unresponsive even while the computer is running.  It doesn't trigger a shut down even when held in for about 20 seconds.  Do you know of a connection between this button and the PSU that could be faulty (and that I could fix, hopefully)?

Open the Power Options control panel. Click Advanced tab and make sure the Power Buttons option is set to "Shutdown".

It's possible the power button is broken. You'll have to contact Dell Spare Parts to get a new one.

Here's another possible work-arournd, if changing the control panel settings don't help:

Run  BIOS Setup and change the Power Option to Reboot after Power Failure.

You'll need to connect the PC to a power strip with its own switch. After you shut Windows down normally, flip the power strip off. To boot, just flip the switch back on. The PC will think that's recovery from a power outage and boot up automatically.

Ron 

Thank you for all your help, Ron!

I like this "work around" a lot.  I changed the BIOS setting to log on after a power outage.  Now I can shut down when the computer is not in use for long periods, and easily log back on.  I just tried it and it works perfectly! 

Leslie

Glad the work-around helps.

Did you check the power button settings in the Power Options control panel?

Ron