Software & Apps Windows 68 68 people found this article helpful What Is a POST Test Card? A POST Test card displays error codes that stop a computer's start-up sequence By Tim Fisher General Manager, VP, Lifewire.com Tim Fisher has 30+ years' professional technology support experience. He writes troubleshooting content and is the General Manager of Lifewire. our editorial process Facebook Twitter LinkedIn Tim Fisher Updated February 09, 2020 Optimal Shop Windows The Ultimate Laptop Buying Guide Tweet Share Email A POST test card displays error codes generated during the Power On Self Test. It's used to identify problems that can be detected as the computer is starting up. What Are POST Codes? POST codes correspond directly to a test that has failed and can help determine what piece of hardware causes problems, regardless if it's the memory, hard drives, keyboard, etc. If the system doesn't encounter an error until later on during the boot process after the video card is activated, then the error can be displayed on the screen. This type of error is not the same as a POST code but instead is called a POST error message, which is a human-readable message. POST test cards are also known as checkpoint cards and port 80h cards. How POST Test Cards Work Most POST test cards plug directly into expansion slots in the motherboard while a few others connect externally through a parallel or serial port. An internal POST test card, of course, requires you to open your computer to use it. During the Power On Self Test, a two-digit code transmits on port 0x80. Some POST test cards include jumpers that let you modify which port to read the code from since some manufacturers use a different port. This code is created during each diagnostic step during bootup. After each piece of hardware is identified as working, the next component is checked. If an error is detected, the bootup process usually halts, and the POST test card shows the error code. You have to know the BIOS manufacturer of your computer in order to translate the POST codes into error messages that you can understand. Some websites, including BIOS Central, maintain a list of BIOS vendors and their corresponding POST error codes. For example, if the POST test card shows the error number 28, and Dell is the BIOS manufacturer, it means that the CMOS RAM battery has gone bad. In this case, replacing the CMOS battery would most likely fix the problem. More About POST Test Cards Since the BIOS can deliver an error message before the video card is enabled, it's possible to experience a hardware problem before the monitor can display the message. This is when a POST test card comes in handy—if the error can't be delivered to the screen, the POST test card can still help identify the problem. Another reason to use a POST test card is if the computer is incapable of making a sound to give the error, which is what beep codes are. They are audible codes that correspond to a particular error message. While they're useful when an error message can't be displayed on the screen, they're not at all helpful on computers that don't have an internal speaker, in which case the corresponding POST code can be read from a POST test card. Few people already own one of these testers but they're not very expensive. Amazon sells many types of POST test cards, many of which cost less than $20 USD.