Publishers of technology books, eBooks, and videos for creative people

Home > Articles

This chapter is from the book

Component Isolation

One of the major difficulties in troubleshooting computer systems is pinpointing the specific cause of a hardware issue.

In Lesson 2, you learned about diagnostic tools and references that can assist you in determining whether an issue is caused by hardware or software. In addition, earlier in this lesson we reviewed the general principles of troubleshooting that Apple advocates.

With that knowledge under your belt, you should be at a stage where you can tell whether you're seeing a software issue or a hardware issue. You should also be prepared to systematically address a troubleshooting issue. We will now look at a method that can aid you in identifying faulty hardware components. This procedure is called component isolation—a technique with which you can accurately and decisively determine the source of hardware issues.

Here's how it works: Using a minimal system, you start up a computer and observe its behavior. Armed with an understanding of the normal power flow sequence (discussed later in this lesson), the symptoms you observe may direct you to add or replace components in a specific sequence until you can determine the hardware component that is causing the issue.

You should not confuse this procedure with randomly swapping modules until a system finally works. As you will see, component isolation works in a much more systematic manner.

You should use component isolation:

  • When you are attempting to isolate intermittent, hard-to-find hardware issues
  • When other approaches have not worked, and you need to make sure that the system hardware is working correctly

How Does Component Isolation Work With Diagnostics?

The last lesson introduced you to a variety of Apple and third-party diagnostic tools. All of these diagnostics can give you indications of defective hardware components, but no diagnostic software is accurate every single time. That's why experienced technicians use multiple diagnostics to verify a particular finding.

Component isolation offers a fail-safe method of confirming that hardware components are functional. It should be considered as a companion technique to diagnostic software.

Understanding the Power Flow Process

The basis for the component-isolation troubleshooting technique is an understanding of power flow within computers.

When a computer starts up, many different activities occur. All of these activities rely on the correct flow of power within the system.

Let's look at a Power Mac G4 (QuickSilver) as it starts up. The following steps are a very simplified description of a complex process. Nevertheless, these simplified steps will assist you in understanding component isolation.

When you press the power button on a Power Mac G4 (QuickSilver):

  1. Power flows through the power cord to the power supply. If the power cord or power button is defective, the system will not start up.
  2. The power supply feeds power to the main logic board. If the power supply or the connection from the power supply to the logic board is defective, the system will not start up.
  3. The logic board in this Mac model feeds power to a CPU card. If the logic board or the CPU card is defective, the computer will not start up.
  4. The logic board feeds power to the RAM as well. If the RAM is defective, the computer will not start up. Instead, you will hear an error sound for defective RAM.
  5. The logic board sends a startup sound or signal to the speaker assembly in the front panel board if the power-on self test (POST) is successful. If this startup sound occurs, you know that the components in this power chain are working correctly. If you don't hear a startup sound, the speaker could be disconnected or defective, or the speaker volume may be turned down or muted altogether.

Starting With a Minimal System

In the description of power flow, we made no mention of hard disk drives. This was intentional, because when setting up a minimal system for the component-isolation technique, you start with only the components necessary to hear a startup sound or see a flashing question mark on the monitor.

You do not need a hard disk drive when testing power flow in a minimal system. The POST does not rely on any components of the Mac OS residing on the hard disk. Likewise, if you have a working power button on the Mac itself, you do not even need a keyboard.

A minimal system is exactly that. For a Power Mac G4 (QuickSilver), for example, the minimal system consists of the AC power supply (including, of course, a power cable), logic board, front panel board, speaker assembly, and CPU with heat sink. All other devices should be disconnected, although it's not necessary to physically remove them from the computer unless they're in the way.

Minimal System Chart

The following diagram is a component-isolation job aid for the Power Mac G4 (QuickSilver and AGP Graphics), showing which components on the logic board must be disconnected and which components must remain connected.

Note that some of the required minimum components are not on the logic board. For example, the CPU and speaker assembly are on the logic board, as you can see below, but the front panel board (which controls the power button) is not.

Component-Isolation Procedure

Students in the AppleCare Technician Training (ATT) program are asked to perform the following procedure to reduce a Power Mac G4 (QuickSilver and AGP Graphics) to its minimal configuration for testing. If you are studying this on your own, the preceding diagram gives you the information you need to complete the procedure, but you should be cautious about going ahead without skilled supervision.

  1. Reduce the machine to the minimal configuration.
  2. Press the power button. You should hear a startup sound. A startup sound means the minimal configuration is working. If you get no startup sound, the logic board is probably corrupted or another module in the minimal configuration is faulty.
  3. If you do not get any sound from the minimal system, verify that the power supply is working according to the instructions in the service manual. (For Power Mac G4 systems, you can find this procedure in the Troubleshooting section under “Power Supply Verification.”) If the power supply does not check out as specified in that procedure, replace the power supply with a known-good component.
  4. If you still do not get any sound from the minimal system, reset the PMU and restart. If the PMU chip reset has no effect, perform a logic board reset by removing all power from the logic board for at least 15 minutes.
  5. If you still do not hear a startup sound, remove the RAM DIMM, reset the PMU and logic board, and restart the system. If you get an error sound signifying a memory error, replace the memory with known-good RAM and restart the system.
  6. If none of these steps has corrected the startup issue, install the video card and connect a known-good monitor. Reset the PMU and main logic board, and restart the system. If you get a flashing question mark but no sound, the issue is probably with the speaker assembly or the front panel board.
  7. If you do not hear an error sound or see a flashing question mark, then replace the CPU with a known-good component, reset the PMU and main logic board, and restart the system.
  8. If the system still does not start up, replace the main logic board with a known-good component. Reset the PMU and logic board, and restart the system.
  9. If you hear the startup sound, install the video card (if it is not already installed) and attach a known-good monitor to it. Restart the system and look for a flashing question mark. If you see that image, the video card is working correctly.
  10. By this stage you should have achieved minimal configuration. (You don't have to reset the PMU and logic board once you have achieved minimal configuration unless another service issue appears.) Reattach the internal hard disk drive to the connector on the main logic board. Restart the system. You should see a normal startup screen for the Mac OS installed on that system.
  11. Continue to add any additional components and peripherals one at a time—with power off, of course. If a service issue appears while adding other components, you should go back to the last stage before the service issue appeared, reset the PMU and logic board, and recheck. By this point, you will have a good idea what the service issue is.

Component-Isolation Exercise

  1. What are the five components of a minimal system for a Power Mac G4 (QuickSilver)?
  2. When you first start up your minimal system, you do not get any sound. What component should you check first?
  3. If your minimal system is starting up correctly, what component do you add first?
  4. You get no startup sound from the system after swapping the main logic board. What components are likely at fault?
  5. Why is it important to check cables?

Component-Isolation Exercise Answer Key

  1. AC power supply, logic board, front panel board, speaker assembly, and CPU with heat sink
  2. The power supply
  3. The video card
  4. The speaker assembly and the front panel board
  5. A bad connection due to a defective cable acts just like a bad component.

Peachpit Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Peachpit and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Peachpit products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by Adobe Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive:

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020