Memory Parity Errors

Memory Parity Errors

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Explanation: During the BIOS POST process a memory parity error was detected within the system memory. Parity checking is used to detect memory corruption between the time that data is written to memory and the time that it is read back. This error message means that there is a problem associated with the system memory itself, usually not your motherboard, but at the onset don’t rule anything out, take one step at a time. Depending on the quality of the system BIOS, you may see some specifics on screen about what part of memory caused the error.

Diagnosis: There are many possible causes of memory parity errors, some of them only related indirectly to the memory, or even having nothing to do with the memory at all. In particular, a memory error at start up is often indicative of a wide variety of possible problems.

Background: Memory is an electronic storage device, and all electronic storage devices have the potential to incorrectly return information different than what was originally stored. Some technologies are more likely than others to do this. DRAM memory, because of its nature, is likely to return occasional memory errors. DRAM memory stores ones and zeros as charges on small capacitors that must be continually refreshed to ensure that the data is not lost. This is less reliable than the static storage used by SRAMs.

Data stored in memory is in the form of either a zero or a one, the standard in a digital system. This in itself helps to eliminate many errors, because slightly distorted values are usually recoverable. For example, in a 5 volt system, a “1” is +5V and a “0” is 0V. If the sensor that is reading the memory value sees +4.2V, it knows that this is really a “1”, even though the value isn’t +5V. Why? Because the only other choice would be a “0” and 4.2 is much closer to 5 than to 0. However, on rare occasions a+5V might be read as +1.9V and be considered a “0” instead of a “1”. When this happens, a memory error occurs.

There are two types of errors that can usually occur in a memory system. The first is called a repeatable or hard error. In this situation, a piece of hardware is broken and will consistently return incorrect results. A bit may be stuck so that it always returns “0” regardless of what is written to it. Hard errors usually indicate loose memory modules, bad chips, motherboard defects or other physical problems. They are relatively easy to diagnose and correct because they are consistent and repeatable.

The second kind of error is called a transient or soft error. This occurs when a bit reads back the wrong value once, but subsequently functions correctly. These problems are, understandably, much more difficult to diagnose, but they are also, unfortunately, more common. Eventually, a soft error will usually repeat itself, but it can take anywhere from minutes to years for this to happen. Soft errors are sometimes caused by memory that is physically bad, but at least as often they are the result of poor quality motherboards; memory system timings that are set too fast; static shocks; or other similar problems that are not related to the memory directly. In addition, stray radioactivity that is naturally present in materials used in computer systems can cause the occasional soft error. On a system that is not using error detection, transient errors often are written off as operating system bugs or random glitches.

The predictability of memory errors, or the rate at which we might be able to calculate them has always been a matter of considerable debate. There’s no dispute that today’s DRAM chips are far more reliable than those of five or ten years ago, however this has given rise to an increase in the number of system BIOS developers that remove error detection support from their BIOS code at the demand of the large system manufacturers. Unfortunately though, all of these factors tend to worsen the memory problems in modern systems rather than resolve them. More DRAM memory is being used today than ever before. Just 15 years ago the typical system had 1 MB to 4 MB of memory, while today’s systems usually have 64 MB to 256 MB and more. Much of this has been caused by the sudden drop in RAM prices in the last three years. Today’s systems are running considerably faster than they used to, with the typical memory bus running from 3 to 20 times the speed of those of older machines we remember. Unfortunately the quality level of the average PC is way down from the levels of 10 years ago. Today we are seeing more hardware related problems then ever before. As we have stated throughout this Web site, you do get what you pay for. If you were to inspect today’s E-Machines and some of the products pushed out the door by the cut-throat assembly houses, you could easily identify that the RAM used is often of marginal quality.

In any event, regardless of how often memory errors occur, they do occur. How much damage they create depends largely on the type of data being handled when the errors occur. If you are playing your favorite game and one of the bits controlling the color of a pixel is inverted from a one to a zero on a screen redraw, who cares. However, if you are defragging your hard drive and the memory location containing information to be written to the file allocation table is corrupted, the issue takes on a whole new importance.

There’s only two forms of  true protection from memory errors; One is to use some form of memory detection or correction protocol, and the other is by purchasing quality components and not neglecting your system. Some protocols can only detect errors in one bit of an eight-bit data byte, while others can detect errors in more than one bit automatically. Others can both detect and correct memory problems, seamlessly.

You can learn more about memory related issues at our Performance Center under Memory.

More Information and Troubleshooting Memory Problems

New or Recently-upgraded Systems

Preliminary diagnostic tests or troubleshooting procedures have identified the memory is suspected of being bad, or the system memory is a possible cause of an unknown system problem.

Diagnosis: There are three common categories of memory problems on a new system. The first is improper configuration, or using the wrong type of memory. The second is incorrect installation. The third is hardware failure of the memory module itself.


  • A word of caution here. Electronic components are susceptible to damage by static electricity, therefore always make sure that when touching or moving components within the case, that you always remove the power plug and always ground yourself by touching the case before touching the component.
  • If you are experiencing parity errors, first check and verify the memory type your machine can use, parity, non-parity, ECC etc. Next, check the memory modules themselves to determine that (A) they are compatible with each other, and that the specifications of those modules meet the requirements of your machine.
  • The typical problems associated with new systems are components not properly seated in their sockets, cables not completely attached, and in the case of systems that have been shipped, shock damage from being dropped. This is aside from a complete component failure. Any one of these could be causing the memory to appear to be bad when it is not.
  • If you happen to have an extra set of memory modules available (and yes, we are aware that not everyone has extra memory modules laying around) that are just like the ones you are trying to diagnose, then try swapping the spare modules in to see if the problem goes away. If your system has two memory modules, remove one and swap them. Rarely will two modules go bad at the same time. If you can swap out the memory modules, do so and if the problem goes away, then put the original memory back in to see if the problem returns. If it returns, you can feel very confident that the original memory was bad.
  • Carefully check the memory modules to make sure they are properly seated in their slots. Sometimes they may appear to be inserted correctly when they are not. Ensure that the modules have been pushed all the way into their sockets and that the spring clips or side clamps have snapped into position to properly to hold them.
  • Check all connections within the PC for any that may be loose, and while you’re in there, check the inside of the case for any potential overheating problems.
  • Make sure that you have used the correct sockets. Motherboards have multiple sockets and putting modules in the incorrect ones will often cause problems. For example, most Pentium motherboards have two to four sockets. If you put the modules in the middle two sockets then you may have created a problem and the PC will not boot. Check the motherboard manual to make sure you have done this correctly.
  • Ensure that the size of memory modules you have selected is supported. Some motherboards will not support certain sizes of modules. Consult your motherboard manual.
  • Make sure that you have not inadvertently combined non-parity memory modules with parity memory modules.
  • Check the technology of the memory you are using. Whether a motherboard supports memory types such as EDO, SDRAM, DDR SDRAM or Rambus for example, depends on the chipset used on the motherboard, as well as how the motherboard itself was implemented.
  • If using SDRAM, make sure you are using the right type. SDRAM comes in several varieties, and some motherboards require only one kind or the other. Some motherboards also require SDRAM that has the “serial presence detect” EEPROM on it.
  • Some 430HX motherboards come with DIMM sockets. Most DIMM form factor memory is SDRAM, but SDRAM will not work in these boards because the 430HX chipset does not support it. These slots are intended for DIMM EDO memory.
  • If your system uses DIMMs, make sure that you are using the right kind. DIMMs come in different voltages, and buffered and unbuffered versions.
  • Many motherboards that support both DIMM and SIMM memory will malfunction if both are used on the board at once. This is because most DIMMs require 3.3 volt power, while SIMMs run at 5 volts. When both are present, the DIMMs are fed 5 volts and problems can result. Try the system with only one type of memory.
  • Some systems require a special BIOS setting to be enabled when using more than 64 MB of memory. If you are trying to use more than 64 MB, check for one of these settings and enable it if necessary.
  • Some PCs use proprietary, special modules; for example IBM’s PS/2 systems. Using industry standard memory in a machine that requires special modules, or vice-versa, will cause problems.
  • There could be a problem with the relative speed of the memory modules compared to the timing settings (memory access timing or wait states) that were entered in the BIOS setup program. Double-check the speed of the modules you are using to make sure it is fast enough. Some PCs will work with slower memory modules, but you may need to increase wait states or slow down the memory timing.
  • Some motherboards will work with EDO memory installed, but only if EDO support is specifically enabled through a BIOS parameter. You may need to boot the system with regular fast-page mode memory to get into the BIOS setup, change the setting to EDO, and then shut the machine down and replace with the EDO memory.
  • There could be something wrong the memory modules themselves. Note that bad memory will often pass the BIOS memory test at boot time, and will also often pass the tests performed by those small module testers that many vendors use. Those tests are very superficial and will not catch all memory problems. If you can, try the modules in another PC that uses the same kind of memory. If you have performed all the checks listed in the points above, and the memory works in another PC, the memory itself may very well be bad. Try to replace the memory and see if the problem goes away.
  • Although unlikely, there could be a problem with the power supply. A bad or failing power supply can cause strange memory errors that crop up because the memory is not getting enough power.
  • There could be a motherboard problem. If double-checking all the settings and replacing the memory does not fix the problem, there may be a bad motherboard, a problem with how it is configured or unseen shipping damage.

Existing Systems

Diagnosis: Sudden memory on existing systems are extremely unusual, as most memory problems occur when a system is assembled or when it is upgraded. A failure on an existing system usually means that the system itself has a problem or that the cause was external to the memory module. Some possible causes are system overheating, power supply problems, external power surges or brownouts and lightening damage. System overheating and power problems are the most frequent causes of hardware failures.


  • Normally the first question we are asked is “Okay, where do I start?”. That’s not as difficult as it might seem. The first step towards sorting out what might be causing your problems is to rule out various components one at a time.
  • Overheat conditions and power supply problems is probably the two largest issues that effect computer performance, ranging from system lockups to unexplained memory errors. With this in mind, the obvious starting point is the fans in your computer. Most computers have between two and three fans, one on the CPU (processor), one in the power supply that you can see from the rear of the case as well as a fan at the front or rear of the case. Unplug the power cord from the system and then open the case. Inspect all of the fans, and use a flashlight if necessary, and make sure they are dirt and dust free. Now reattach the power cord and turn the unit on. Are the fans working? If they aren’t, or they appear slow, replace them.
  • Next, and this depends largely on the age of the unit, make certain that the inside of the system case is relatively clean and dirt and dust free. If it’s dirty and dusty inside, this will translate into problems sooner or later. If necessary, leave the case cover off and take the case to a local shop and have him blow the dust out of it. Just be careful not to bump or move anything inside.
  • Locating power related problems can be difficult, however if your area is subjected to more than one or two power surges or brownouts a year, or frequent lightening etc, you may want to consider replacing the power supply and then purchasing an inexpensive surge protector. Your system requires adequate metered power, and if it isn’t available you will have problems. Obviously we can’t see how many drives you have or anything else, but if your system appears sluggish, take the unit to a local PC repair shop and ask them to check the power supply.
  • The next step would be to make sure that the systems BIOS settings have not been changed. Checking this will require the manual for the system’s motherboard (main board) and some familiarity with computers. If you feel uncomfortable, don’t do it. Double-check the settings that are directly related to the system memory and make sure that they are correct.
  • Check all of the connections inside of the PC as something inside the PC may have come loose. 
  • If the memory modules being used in the PC do not use the same metal (gold or tin) as the sockets they are in, and it is possible over time for a chemical reaction to develop that can lead to poor contact and eventually, memory problems. This will typically take months or even years to show up. If you suspect this problem, power down and unplug the PC and take out one memory module. If its pins are gold and the socket is tin (silver color) or vice-versa, this may be the problem. If so, remove all the modules and clean them and the socket. You will find more information about troubleshooting memory, including how to properly clean the modules here: Troubleshooting Memory Problems
  • If you would like to run some software memory tests, then visit our testing summary pages here: Using Diagnostic Software to Detect or Confirm Memory Problems
  • Always keep in mind that there could be something wrong the memory modules themselves. Bad memory modules can (and often will) pass the BIOS memory test at boot time, and can often pass the tests performed by some small module testers that many vendors use. Those tests are not very thorough by any means and will not catch all memory problems. If you can, try the modules in another PC that uses the same kind of memory. If you have performed all the checks listed in the points above, and the memory works in another PC, the memory itself may very well be fine.
  • There may be a problem with a component on the motherboard, or another part of the PC.

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