Windows 7 : Troubleshooting Hardware Components (part 1) - Understanding the Boot Process, Troubleshooting the Power Supply Unit

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1. Distinguishing Hardware Failures from Software Failures

When a computer system fails, you should first try to determine whether the failure is a result of software or hardware errors. This determination isn't always easy. Though some hardware-related failures are simple to distinguish from software-related ones, others (for example, those resulting from a damaged memory module) exhibit behaviors remarkably similar to software-related failures.

In general, however, the following rule applies to failures caused by faulty hardware.

A system failure is caused by a hardware problem when one of the following occurs:

  • The failure occurs before the operating system loads.

  • The failure occurs randomly, in a way that suggests no relation to any particular software activity.

If you suspect that a system failure is caused by a hardware problem, you can use the information in this lesson along with the tools described in Lesson 1 to diagnose the particular nature of the problem.

2. Understanding the Boot Process

If a hardware device is not functioning, this problem often reveals itself before the operating system loads. For this reason, when you are troubleshooting hardware issues, it is important to understand in a computer boot sequence the steps that precede the start of the operating system. If you can observe at what point the failure occurs, familiarity with this sequence can help you pinpoint the particular component that is failing.

The following steps summarize the boot sequence, up to and including the load of the operating system:

  1. Power on.

    During this phase, the power supply feeds power to the motherboard and the CPU (chip).

  2. Perform instructions contained in the BIOS.

    Once the CPU has power, it immediately starts executing the instructions that are written in the BIOS. The BIOS is an example of firmware, or low-level software that works closely with hardware. A computer's BIOS contains the processor-dependent code that is responsible for testing basic hardware functionality of the computer and for passing control to the boot device.

    The BIOS also contains software interfaces to hardware that enable the operating system to use features such as power management, virtualization, hot swapping, and booting from universal serial bus (USB) devices.



    EFI is an advanced replacement for BIOS that is beginning to appear in some new computers. Whether a computer uses BIOS or EFI for its firmware, the essential role of this firmware in the computer's boot process is the same.

    During the boot phase, the instructions in the BIOS consist of two steps:

    1. Perform the power on self test (POST)

      The POST is the hardware check that is performed by the BIOS as soon as the computer is turned on. When the POST detects a hardware error such as a failed video device, it signals the error with a beep code indicating the type of failure detected.

    2. Read instructions on the boot device

      The second function performed by the BIOS is to pass control to the boot device and read the instructions on that boot device. The boot device should be the device on which the operating system is stored. Typically, this boot device is an internal hard disk, but in the BIOS Setup program, you can specify the order of devices that you want the BIOS to investigate for boot code.

  3. Operating system loads from boot device.

    If the boot sequence fails to reach this point, the problem can be the result of an incorrectly configured selection of boot device in the BIOS Setup program, of a faulty Master Boot Record (MBR) on the hard disk, of a failed driver (typically for a SCSI hard drive), or of a hardware failure.

    It is worth mentioning that if a computer crashes after the operating system begins to load from the boot device, the failure is somewhat more likely to be the result of a software problem than a hardware problem. But this is not a rule; hardware-related crashes can occur at any time.



You might need to upgrade your BIOS to enable certain features such as booting from a USB or network device.



When troubleshooting, always begin by taking the overall least risky, costly, and difficult action that can help you narrow down or identify the source of the problem. Then, if you need more information to identify the problem, take the overall next-least risky, costly, and difficult action, and so on.

3. Troubleshooting the Power Supply Unit

The power supply unit converts AC current from the wall outlet into DC current at the proper voltages needed by various computer components such as the motherboard.

The following section provides a set of basic strategies for troubleshooting power supply problems.



Do not touch internal components when a computer is plugged in. You can electrocute yourself or seriously damage the computer. Note also that computer circuits are extremely sensitive to static electricity, even at levels that we can't feel. Before you touch any components, always ground yourself by first touching the metal structure of the computer case.

The computer appears dead. (There are no fans, lights, sounds, or signs of movement when you attempt to start it.)

  1. Verify that the wall outlet is working.

  2. Verify that the power cords are properly attached to the wall outlet, to the computer, and to the motherboard. (Remember that most modern motherboards require two power connectors.)

  3. Verify that any internal power switch is turned on. If such a switch exists and is turned on, and if the power supply works in another computer, replace the switch.

  4. If your power supply has a voltage switch, verify that the switch is set to the proper AC voltage for your country.

  5. If the previous steps do not uncover the source of the problem, replace the power supply.

The computer freezes before the operating system starts.

  1. Compare the power requirements of your devices with the power capacity of the power supply unit. Verify that the power supply unit provides the wattage necessary to power all the computer devices in your computer. If not, replace the power supply with a more powerful unit.

  2. Test with a multimeter to determine whether the power supply unit is supplying correct and consistent voltage to the machine. If not, replace the power supply.

The computer suddenly shuts off at unpredictable moments.

  1. Verify that the power supply unit fan is working. If not, you can replace just the power supply fan.

  2. Verify that the motherboard fan is working. Replace this fan if necessary.

  3. Run Windows Memory Diagnostic to check your RAM for hardware faults.

  4. Run motherboard diagnostic software to check the functionality of the motherboard. To obtain this software, consult the motherboard manufacturer.

  5. If the previous steps do not uncover the source of the problem, replace the entire power supply unit.

The power supply unit is making a loud, continuous noise.

Replace the power supply unit.

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