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:
-
Power on.
During this phase, the power supply feeds power to the
motherboard and the CPU (chip).
-
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.
Note
NOTE EXTENSIBLE FIRMWARE INTERFACE
(EFI)
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:
-
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.
-
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.
-
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.
Tip
EXAM TIP
You might need to upgrade your BIOS to
enable certain features such as booting from a USB or network
device.
Note
BASIC TROUBLESHOOTING
STRATEGY
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.
Caution
UNPLUG YOUR COMPUTER BEFORE OPENING THE
CASE!
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.)
-
Verify that the wall outlet is working.
-
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.)
-
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.
-
If your power supply has a voltage switch, verify that the
switch is set to the proper AC voltage for your country.
-
If the previous steps do not uncover the source of the
problem, replace the power supply.
The computer freezes before the operating
system starts.
-
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.
-
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.
-
Verify that the power supply unit fan is working. If not,
you can replace just the power supply fan.
-
Verify that the motherboard fan is working. Replace this fan if
necessary.
-
Run Windows Memory Diagnostic to check your RAM for
hardware faults.
-
Run motherboard diagnostic software to check the
functionality of the motherboard. To obtain this software, consult
the motherboard manufacturer.
-
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.
