Troubleshooting Guide for A20 Errors

A20 Error illustration

The A20 error displays during the Power On Self Test (POST) process very soon after the computer is first started. The operating system has not yet loaded when this error message appears.

The A20 error message may appear in many different ways, but these are the most common:

Error A20
A20 Error

What is the Cause of an A20 Error?

The "A20" error is reported by the POST when it detects a problem with the keyboard or the keyboard controller located on the motherboard.

While it's possible that an A20 error could apply to something else, it's very unlikely.

How To Fix the A20 Error

  1. Turn off the computer if it's on.

  2. Disconnect the keyboard from the PC.

  3. Verify that the pins on the keyboard connector are not bent. If they are, you can attempt to straighten the keyboard connector pins yourself and try the keyboard again.

    To do this, first remove any dust or debris from the end where you see the pins. Then, with a paperclip or something else, like a pen, bend the connector pins to the point that they look straight again.

  4. Verify that the pins on the keyboard connector do not appear broken or burned. If any do, replace the keyboard.

  5. Also, verify that the keyboard connection on the computer does not appear burned or damaged. If so, the port may no longer be usable.

    Since the keyboard connection is located on the motherboard, you may have to replace the motherboard to resolve this issue. Alternatively, you could purchase a new USB keyboard.

    Shop for USB Keyboards at Amazon

    Using a USB keyboard would circumvent the need to use the standard keyboard port on the computer at all.

  6. Plug the keyboard back in, making sure it is plugged firmly into the correct port.

    If you're still having troubles at this point, make sure the PS/2 port is clean and wiggle the connection around as your press it in. It's possible you may end up bending a pin just right so that the cable will connect correctly.

  7. If the A20 error persists, replace the keyboard with a keyboard that you know works. If the A20 error disappears, the cause of the problem was with the original keyboard.

  8. Finally, if all else fails, there may be a hardware issue with the keyboard controller on the motherboard. If this is the case, replacing the motherboard should solve this problem.

  9. You might also be able to check that the controller chip is firmly in place. If it's socketed, it's possible it just needs to be pushed in further.

What the A20 Error Applies to

This issue applies to any PC keyboard hardware. The operating system is not involved in generating this error message, so you could receive it no matter what OS you might be using.

Some software programs may use an A20 error for something completely unrelated to a keyboard or keyboard controller issue. Stan is one example, where an "Error A20" means a video is unable to stream.

More Information on the A20 Error

Some computers may let out a sequence of noises to indicate an error. These are called beep codes. See How To Troubleshoot Beep Codes if you need help finding the BIOS manufacturer and/or help to understand what the beep codes mean.

It's also possible to identify the A20 error through a POST code using a POST test card.