CMOS Checksum Error

I am running Windows XP SP2 …. the machine is approx 4 years old. Recently when I log on (after unlpugging computer from mains power supply) I get a screen message saying CMOS Checksum Error … the computer askes me to press F1 and it loads OK excpet that the date and time shown as 31st December 1999. ???? Normally I shutdown the computer but leave it plugged into the power suppy and have had no problems. Is this problem to do with the BIOS Battery ? Any help or suggestions would be appreciated.

What you describe is typical of a bad battery and 3 - 4 years is when many start to fail. When the battery does not have enough power to maintain voltage on the "volatile" CMOS memory module, all data held in that memory is lost. That data includes the date and time, RAM count, hard drive specification and boot sector location information. When it is lost, the BIOS is "reset" back to out-of-box-state, as programmed ("flashed") into the BIOS chip's firmware module.

Fortunately, they are very easy to replace and for just a couple dollars at your local home improvement or discount store battery/watch/camera counter, it is certainly an inexpensive task that eliminates the battery from the equation.

Most batteries used in the last several years are the common CR2032 3V Lithium wafer-type batteries - but check your motherboard and/or current battery to be sure.

To replace the battery, power down AND UNPLUG the computer from the wall, then open the side of the case and locate the battery on the motherboard. WARNING - BEFORE reaching, and frequently thereafter, touch bare metal of the case to discharge any static electricity in your body to prevent destroying any ESD sensitive devices.

Inspect the interior of the case. Clean out any heat trapping dust, dirt, hair, dander, microscopic critters that eat dander, and the microscopic droppings of those microscopic critters. Allergy sufferers, wear a mask. Remember the warning above. Give each fan a little "flick" to make sure it spins freely. Become familiar with the cabling, both power and data cables.

It usually takes a couple sturdy thumbnails and a strategically placed tongue to pry the batteries out. Some battery holders have a small tension clamp that must be pushed requiring a bit of dexterity. I do not recommend using any metal object to pry it out as one tiny slip can cut across several circuit runs on the motherboard - with expensive consequences. I have found some neat plastic and wooden (non-conducting) poking, prodding, and prying tools in the lady's finger and toenail tools section of your local hardware, err cosmetics counter. Note the battery polarity orientation (usually + side up) before removing it.

Many battery counters will recycle the old batteries for you so take the old battery with you. Radio Shack does (and they recycle UPS batteries too!).

Do NOT touch the new battery with your bare hands - skin oils attract dust, and promote corrosion. I put a clean sock over my hand. Fortunately, inserting the new battery is easy - just make sure you insert it in the correct polarity orientation. Inspect the inside of the case again, this time, making sure (I press on each one) every power and data connection is still tight and secure, and wires are routed to minimize impact on the desired front to back air flow.

Upon first boot after inserting new battery, you will probably get the checksum error again - that's okay - remember the BIOS has been reset. Enter the BIOS Setup Menu and set the date and time, make sure your drives are correctly identified and the boot order is right. Check the temperatures under the PC Health section - temperatures should never be high in the BIOS Setup Menu. Then Save and Exit. If you don't "Save", you will get your checksum errors again.

It will reboot and you should be good to go. Let us know how it goes.

Normally I shutdown the computer but leave it plugged into the power suppy and have had no problems.

And that is all you normally need to do. It is not typical to unplug the computer when you are done using it, except if you expect to be away for an extended period, or during a lightning storm or some other anticipated abnormal electrical event.

Every one is using an UPS with AVR (automatic voltage regulation), right? If not, you should be. Surge and spike protectors are little more than simple, but expensive, extension cords.

WOW !!! What a helpful guide you have produced for me Digerati. I am off later to but a new battery and fingers crossed, I will follow your instructions to the word - clean socks at the ready, hoover, glasses, prodding tools, steady hand and a hopefully it will sort things out. I will let you know how I get on, thanks again for your help

If not too late, I will dig up my "hoovering" instructions in just a couple minutes.

Okay - here's the "beta" of my canned text on using an air compressor, or a vacuum to clean out your computer. Be sure to let me know if something need clearing up.
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You often hear folks insist you do not use an air compressor to blast out the insides of your computer and power supplies because they are too powerful, spew out moisture, rust, oil and debris, and possible damage from physical contact with the nozzle.

And others will insist you can never use a vacuum ("hoover" in the UK, and elsewhere I am sure) because of static generation, and damage from the contact with the nozzle.

These are all valid concerns that must be addressed. Here's my take, plus I'll address a couple other serious safety concerns. Remember, anything that plugs into the wall can kill!

First, the air compressor. I use one all the time to clean computers, TVs, and audio electronics. Just about every compressor allows you to throttle down the output side pressue, I set mine to 80psi. Do note that cake makers and artists use air compressors all the time when they use their air brushes for detailed air brush painting - so too much pressure is not a problem.

There are some serious concerns with a compressor, however. Some spray a fine oil mist - great for automatically oiling nail guns and other pneumatic tools, not so great for electronics. So if you are in the market, make sure it is an oiless compressor - all the major brands make them.

See my post here for more details and note the comments about using an in-line filter to prevent any moisture inside the tanks, or other contaminates from spraying onto the electronics.

So, assuming you have an oiless compressor and an in-line air filter, all you need then is some non-conductive object to hold the fans still. I buy bags of popsicle sticks at my local Hobby Lobby store. As HFCG mentioned, use the stick to hold the fan stationary while blasting with compressed air (from compressor or canned air). The reason why is because the compressed air can easily cause the fan to spin much faster than designed limits and that can damage the bearing, which then will result in premature failure. So while it is cool to watch them spin up, don't do it!

Using some sort of forced air is best at removing heat trapping dust, so if compressed air is not possible, vacuuming is the next best thing. However, it is VERY DANGEROUS to use a vacuum cleaner to clean the inside of a computer. The reason why is because the air and dust particles zipping past the nozzle of the vacuum can (will!) generate a large potential of static electricity in the nozzle. These electro-static charges easily build up in the human body. The "potential" (voltage) of the static electricity in humans can easily exceed 30,000 volts!! The threshold for human awareness is only around 3,500 volts, depending on many factors, including dryness of the skin, sensitivity, etc.

Electro-static discharge (ESD) sensitive devices used in sophisticated electronics, such as computer processors (including CPUs and GPUs) and memory integrated circuits (ICs), can easily be destroyed by static with potentials as little as 30 volts! This means that a static shock can totally destroy a sensitive device without you even being aware a discharge occurred!

Because of this static, it is absolutely essential precautions be taken to prevent static buildup and damage from ESD, as well as physical damage from banging the nozzle into fragile devices, or scratching the motherboard. With the computer powered down and unplugged, and with a soft brush in one hand and the vacuum nozzle in the other, extend a finger on your nozzle holding hand out past the tip of the nozzle and plant that finger on bare metal of the case. Then simply use the brush to sweep the dust into the vacuum. With your finger touching metal, and your hand wrapped around the nozzle, no static can build up, no ESD damage can occur. Every time you reposition the nozzle, touch metal with an extended finger before moving in close. As with the compressed air, resist the temptation to see how fast you can make the fans spin.

The key is to remove the "difference in potential" between the two conductors - you, and the device.

Notice I did not say you have to "ground" the static. I said you must "remove the difference in potential between the two conductors". What this means is that it is not necessary to have the computer case connected to the facility ground to effectively eliminate static. Just as you can eliminate static in your body by touching the doorknob of an ungrounded, wooden door (or the dog's nose ), simply touching the case will work just fine. When you touch the case, you equalize and eliminate that difference, thus eliminating the chance of a static discharge to a sensitive component (think of a "floating" ground - such as used in avionics - airplane electronics). Continually touching the bare metal case (through a wrist strap, or dangling pinky finger) will prevent static caused by wiggling around in your clothes, moving on the carpet, sliding on the chair cover, etc. from ever building up. If you can't stay in constant contact with the case, frequently touching the case will discharge any build up before reaching damaging potential, and is generally considered a sufficient precaution.

So always before reaching inside the case, make sure you equalize any difference of potential between you and the computer by discharging yourself to the bare metal case of the computer.

Now, notice I keep saying to unplug the computer. Some folks may have heard it is okay to leave the computer plugged in. THIS IS WRONG!!!! Ask any formally trained technician. There are several reasons - all related to safety.

First, anything that plugs into the wall can kill!!! So, don't become a "Darwin Award" candidate by assuming your power supply is working and wired properly. Even the best can and do fail, and sadly, many are no-name generics made in child slave labor camps in China or elsewhere. So ALWAYS unplug before reaching inside.

The second reason is that the ATX Form Factor standard calls for +5VDC @ 2 amperes across several motherboard circuits whenever the computer is in Standby Mode. Standby Mode is enabled whenever the power supply is plugged into an AC power source (the wall - or UPS), and if equipped, the master power switch is turned on. If the PSU does not have a master power switch, and many PSUs do not, Standby Mode is enabled whenever the power cord is connected. Standby Mode allows for such features as "soft power control" which allows the case's front panel power switch to power up the computer, and other features, such as Wake on LAN, Wake on Modem, Wake on Keyboard, and Wake on Mouse. For the Wake on LAN and Wake on Modem features to work, the +5VDC (normally designated as "+5Vsb") is present across the PCI bus (which includes the expansion slots) whenever the power supply is plugged in, and if equipped, the master power switch is set to on.

So, ALWAYS, unplug - especially before installing or removing any hardware, such as memory modules and cards.

For small projects like keyboards, mouse opticals, etc. the cans of compress gas for dusting work great. Note they are NOT cans of "air" but gases such as difluoroethanes. They can be fatal if inhaled. And like butane from a can, the gas from these gas dusters can be discharged in the form of super-cooled liquid that can cause frost-bite, or worse if inhaled. Read the labels and hold the can upright.

Whether from an air compressor, or canned dusting gas, do it outside so it is not immediately sucked back into the computer by the cooling fans.

I inspect my internals one a month and clean when necessary. My wife's computer in the same room has a washable filter, mine does not. I have to take my PC outside 3 times as often as hers. I will never buy a case without an easy to remove, washable filter.

Well I have just bought me a battery … but before I start I re-booted the computer, checking the BIOS settings but was somewhat confused when the BOOT set up was showing as 1. Removable Device - legacy floppy drive 2. IDE Hard Drive 3. ATAP I CD-ROM 4. Other Boot Device …. Is this the order will have to re-set after changing battery ? I understood that the 1st boot from was the hard drive ? Your advice would be much appreciated.

Those are default settings when the BIOS is reset - so you don't have to remember them. That just means it will look for a bootable floppy first, if it can't find one, it will move to the IDE hard drive controller and look for a boot drive, hopefully find yours, and boot. You can leave it as is, but I always change mine by habit from the old days when viruses could only infect your computer by being on a floppy disk and the computer reboots. Sadly, since the only way to move files between computers was by sneakernet, many folks would forget a floppy was in the drive, reboot, and suddenly the virus infected a new machine. So for security, I always changed those settings so viruses would not get us, but also so a bad guy could not insert a bootable floppy, boot to a DrDOS and take over the machine, and everything on it! I have mine look to my boot drive first - that speeds up the boot process by a couple seconds too - never a bad thing. If I want to boot to my CD, I change it CD first, and it will prompt to push a key within a few seconds for CD, don't press a key and moves to the HD. Your main thing is to set the date and time, then check to see if your drives have been properly identified (it may call your DVD a CD, but that is okay now). Then it is essential you Save and Exit. Then next time you reboot, it should have the correct checksum in memory and it should boot fine. One thing - on some motherboards, that +5Vsb voltage may be used to to help keep the CMOS alive, easing the burden on the battery - depends how the maker decided to do it. Frequent long periods with a load on the battery (computer unplugged from wall) may cause the battery wear out early.