When the computer is booted, the basic input/output system (BIOS) performs a basic hardware check. This check is called a power-on self-test (POST).

The POST, as shown in Figure 1, checks whether the computer hardware is operating correctly. If a device is malfunctioning, an error or a beep code alerts the technician of the problem. Typically, a single beep means that the computer is functioning properly. If there is a hardware problem, a blank screen might appear at bootup, and the computer emits a series of beeps. Each BIOS manufacturer uses different codes to indicate hardware problems. Figure 2 shows a chart of beep codes. The beep codes for your computer might be different. Consult the motherboard documentation to get the beep codes for your computer.


When troubleshooting a computer problem when no video is available, you can use a POST card. A POST card is installed in a port on the motherboard, such as PCI or PCIe. When the computer is started, and errors are encountered, the computer issues a code that is displayed on the POST card. This code is used to diagnose the cause of the problem through the motherboard, BIOS, or POST card manufacturer.