PC Hardware

POST-Power-On Self Test

Every time a PC is turned on or reset using the Reset button or Windows Restart command, the computer is rebooted and reset to its basic operating condition. The system BIOS program starts by invoking a special program (stored on a ROM chip) called the POST (power-on self test). The POST sends out standardized commands that check every primary device (in more technical terms, it runs an internal self-diagnostic routine).

The POST has two stages:

  • Test 1 occurs before and during the test of the video.
  • Test 2 occurs after the video has been tested.

This division determines whether the computer will display errors by beeping or showing them on the screen. The POST does not assume the video works until it has been tested. The POST does assume that the speaker always works, but in order to let you know that the speaker is working, all computers beep on startup. Depending on the BIOS type, the POST might also sound a single beep when it's done, to let you know the boot process was successful. If something goes wrong, the POST sends a series of beep codes to let you know what the problem is or where to start looking for it.

Beep Codes Before and During the Video Test

The purpose of the first POST test is to check the most basic components. The exact order, number of tests, and error states will vary from product to product. In a healthy system, the POST reports by using a series of beep codes and screen messages to convey that all components are working. Then it transfers control to the boot drive, and the operating system is loaded. The POST is a good indication that the hardware is in working order.

If a problem occurs, the POST routine attempts to report the problem. This is also done by beep codes and (if possible) screen prompts. Some error codes are specific to chip sets or custom products, and the exact message and its meaning can vary from system to system. (See the POST code references in the system manual that shipped with the PC or the motherboard to obtain references for detailed error messages and beeps.) The following table lists the basic beep codes for AMI and Phoenix BIOSs.

Number of Beeps Possible Problem
1 DRAM refresh failure
2 Parity circuit failure
3 Base 64 KB or CMOS RAM failure
4 System timer
5 Processor failure
6 Keyboard controller or Gate A20 error
7 Virtual mode exception error
8 Display monitor write/read test failure
9 ROM BIOS checksum error
10 CMOS RAM shutdown register failure
1 long, 3 short Conventional/extended memory test failure
1 long, 8 short Display test and display vertical and horizontal retrace test failure

Troubleshooting After a Beep

After a beep code has been recognized, there are a few things you can do to troubleshoot the error. The following table suggests some solutions. Keep in mind that in many cases it can be less expensive to replace the motherboard than to replace a chip.

Problem Solution
RAM refresh failure
Parity error
RAM bit error
Base 64-KB error
Reseat and clean the RAM chips.
Replace individual memory chips until the problem is corrected.
8042 error (keyboard chip)
Gate A20 error
Reseat and clean keyboard chip.
Check operating system.
Replace keyboard.
Replace motherboard.
BIOS checksum error Reseat ROM chip.
Replace BIOS chip.
Video errors Reseat video card.
Replace video card.
Cache memory error Reseat and clean cache chips.
Verify cache jumper settings are correct.
Replace cache chips.
Any other problems Reseat expansion cards.
Clean motherboard.
Replace motherboard.
Many computers will generate beep codes when the only problem is a bad power supply! Turn the computer off and on three or four times to see if the same beep code is generated every time. If so, it's probably a legitimate beep code that concerns the hardware and not the power supply.

Since early 1996, some BIOS programs have eliminated many beep codes. However, beep codes can still be found as part of the A+ Certification exam.

Error Messages-After the Video Test

After successfully testing the video, the POST will display any error messages on the screen. These errors are displayed in one of two ways: numeric error codes or text error messages.

Numeric Error Codes

When a computer generates a numeric error code, the machine locks up and the error code appears in the upper-left comer of the screen. The following table lists some common numeric error codes, but it is a good idea to check the manual before beginning repairs based on a beep code or error message.

Error Code Problem
301 The keyboard is broken or not plugged in.
1701 The hard disk drive controller is bad.
7301 The floppy disk drive controller is bad.
161 The battery is dead.
1101 The serial card is bad.

Text Error Codes

BIOS manufacturers have stopped using numeric error codes and have replaced them with about 30 text messages. Instead of numbers, you get text that is usually, but not always, self-explanatory.

How Bad Is It?

There are two levels of error codes during POST: fatal and nonfatal. As the name implies, fatal errors will halt the system without attempting to load the operating system. Memory problems or a faulty disk or display adapter are examples of fatal errors. Nonfatal errors like a "missing" floppy disk drive will still result in the system attempting (and often succeeding) to load the operating system.

In most cases, the POST procedure does a good job of testing components. If it gives a clean bill of health to the hardware, its failure to boot will often lie in the operating system. You can use a bootable floppy disk in most cases to access the hard disk drive, or boot Windows using the Safe Start approach (press the F8 key just after the POST completes) and check for conflicting settings.

POST Cards

More difficult to resolve is a hardware problem that keeps the POST from issuing any report at all. When you face one of these, you will find that this is where a POST card earns its keep. These special diagnostic expansion cards monitor the POST process and display all codes (usually in two-digit hexadecimal format) as the system runs the POST. The technician can then decode this information using the manufacturer's manual. More advanced models can also be used to run advanced series of tests to isolate erratic problems.

When choosing a POST card, be sure that it will work with the types of machines you plan to test. Most are based on the ISA slot and work with most Intel CPUs. That means they should help with AT and later-based PCs that use the x86 processors. Basic models give only POST codes. More advanced models also can check DMA (direct memory access), IRQ (interrupt request), and port functions. Some come with fancy diagnostic software. The more features, the higher the price tag. But a POST card will save a lot of time and frustration, making it a worthwhile addition to any PC toolkit.