The iMac Firmware Problem Solver

Giles Kennedy, who I met over at the CRT iMac forum at due to this firmware issue, is the author of the document below. This is v2.1, 23 Oct 2004 of Giles' famous iMac firmware problem solver. After Giles successfully rescued an iMac CRT (with all of us cheering him on, and holding his hand, and toasting), he suggested that someone should document the procedure for others who fall into the trap. Since there were no takers, he took the bull by the horns and did it himself.

This document has helped countless people since Giles first offered it publicly. It was originally created in Microsoft Word (Feb 2004) and made available for download. A few die-hard Mac users still shy away from using Microsoft products, and didn't know what to do with a Word doc. So, I used Microsoft Word's 'Save as html' feature to convert it (not the best solution, I know, but it sure beat typing it) and then I put it here on my site for the benefit of those Mac users who were not willing (or able) to open a Word document.

Since then Giles has also created an html version and put it up on his site. I will do my best to keep this 'mirror' of Giles' page updated for those who have bookmarked it. However, to check for the latest revisions please go here:

Note: I updated this page on November 20, 2005 to correct broken links to the iMac CRT discussions area that were caused by Apple changing their discussions web software. Some additional thoughts on the subject have been kindly provided by others. That additional information may be found here.

On behalf of the Mac Community, we thank all who have shared their knowledge.


I didn't update the firmware on my iMac before trying to install OS X 10.2 or higher on a slot-loading iMac: how do I fix resultant bootup/screen problems?

Disclaimer: while the information here is presented in good faith, we take no responsibility for errors or omissions


I Preliminaries

II Introduction to the problem and description of its symptoms

III Resolving the problems - overview

IV Resolving the problems - working round S0 - S3

V Resolving the problems - upgrading the firmware

VI Suggestions if you canít get a usable screen display

VII Tackling symptom S4

VIII Closing remarks

Appendix: Useful startup tricks

I Preliminaries

1. This article is applicable only to slot-loading CRT iMacs with firmware (Boot ROM) versions prior to 4.1.9. Tray-loading iMacs have different architecture and firmware. (How to differentiate between models)

2. Usually the problems are caused when attempting to install or boot OS X 10.2.x (Jaguar), 10.3.x (Panther) or higher from an install CD. Many users have experienced similar symptoms and managed to get their iMac working again.

3. The most important thing is not to reset the PRAM until the firmware is updated, as doing so can make the problems much worse.

4. This article deals only with resolution, which is usually possible. The detailed causes of the problem are discussed elsewhere on the internet.

5. Note that OS X installs successfully on a proportion of slot-loading iMacs with pre-4.1.9 firmware. Although you may not be experiencing the symptoms listed below at the moment, it is strongly recommended that the firmware is updated to 4.1.9 as soon as possible as problems may arise at any time. It may first be necessary to install or reinstall (or upgrade an existing OS 9.0.x system to) OS 9.1 or 9.2.x. (Note that it is quite OK to have more than one system installed at the same time, e.g. OS 9 and OS X.) See para. 27 for advice on upgrading the firmware.

6. The firmware (Boot ROM) version can be checked using Apple System Profiler (Apple menu), but the best way is to boot into open firmware mode. Restart the iMac and hold down <Option> <Apple> <o> <f>. Make a note of the build version and date ("4.1.9f1 BootROM Built on 09/14/01" is the latest). Type "mac-boot" without the quotes to continue with a normal bootup.

7. For further advice on the issues discussed here, contact Apple Support, post a new topic in the iMac(CRT) Usage discussion forum, or check for further suggestions at the iMac SL firmware problem solver site.

II Introduction to the problem and description of its symptoms

8. OS X 10.2 (or higher) plus a slot-loading iMac with pre-4.1.9 firmware is a recipe for an array of characteristic symptoms. The problems are usually caused during the install process when the installer restarts the iMac and boots OS X from the CD. The same problems can be caused in other situations, e.g. booting OS X from a Utilities CD such as TechTools or DiskWarrior. (Similar problems have also been reported very occasionally even when OS X has been nowhere near the iMac: such as when application (probably a game) or utility has attempted to set the video adaptor to an invalid settings, and the iMac has subsequently shut down improperly).

9. When the OS X install CD is implicated, the troubles begin as follows:

S0 -- The first thing that usually happens is that after booting OS X from the CD the installer informs the user that the firmware needs to be updated and prompts the user to click to Cancel the installation or Restart the iMac. At this point the iMac freezes (typically with a grey screen). A forced power-off is required (hold the main power button for a few seconds). The iMac then won't boot, the whole thing seems to be completely dead, and the CD won't come out. Actually the iMac is displaying symptom S3 below.

10. Once the user has overcome S0, a number of other symptoms usually appear:

S1 -- iMac starts up but with blank display. You should hear the chime, the HD spin up, and the chatter of the HD as the OS loads. (If your iMac didn't perform a proper shutdown the last time, in order to complete a successful bootup you may need to press the <Return> key (sometimes labelled <Enter>) to a clear a dialog telling you the hard disk is going to be checked, and again when the check is complete.)

S2 -- iMac starts up normally but with dim or strangely coloured display.

S3 -- If the user does a restart, the iMac shuts down but then appears to be dead. Similarly, if the user shuts the iMac down and then tries to power the iMac on it seems to be dead. Pressing the power button again does not help. The power LED shows green but nothing else happens and the screen is blank. There is usually no startup chime. You may hear a very faint clicking (seemingly coming from the hard drive).

S4 -- iMac turns itself off 7 - 10 seconds after power-on, even if the power cord was pulled and replaced. You should hear the chime, the hard may drive spin up briefly, and the screen (or external monitor) may come on momentarily. This symptom usually follows a PRAM reset.

III Resolving the problems - overview

11. All of these symptoms are resolved by updating the firmware to version 4.1.9. However, there are a number of steps involved in achieving this, as detailed below, and for practical reasons is may be a couple of days before you are able to complete the process. You need to work round the various symptoms you are experiencing. The symptoms are obstacles on the path to your goal of a successful firmware update.

12. The first thing to do is to disconnect all peripherals (printers, scanners, etc.) except keyboard and mouse, to minimize the risk of other hardware interfering with the already complex problems.

13. This article deals first with symptoms S3, S0, S1, S2, and then upgrading the firmware. The basic resolution process is as follows:

    • (where necessary) get the OS X install CD out of the iMac
    • get the iMac to boot
    • get some sort of usable screen image
    • (where necessary) install or reinstall or update the OS to 9.1 or 9.2.x, then boot the newly installed 9.1 or 9.2.x
    • obtain and run the firmware updater
    • (where required) update OS 9 to the latest version, 9.2.2, via 9.2.1
    • install OS X

14. Finally there are a few comments on S4. Be aware that while a few simple steps may enable you to recover from S4, often more advanced procedures are required that require some technical competence in electronics and are beyond the scope of this topic. On the other hand, while replacement of the iMac's logic board is a valid solution to the problem sometimes recommended by repair shops it is expensive and usually unnecessary - it's overkill. Replacement of other hardware components (such as the power supply or power/analog video board) is unlikely to help. You may need to check out suggestions on this webpage.

15. We repeat - do not reset the PRAM as this usually leads to symptom S4. If the iMac is already at S4 it probably won't make things any worse but it won't make them any better either. Also note that replacing the internal logic board battery causes a PRAM reset. Once the firmware is updated then you are safe to reset the PRAM.

16. Finally, try to avoid rebooting the iMac unnecessarily.

IV Resolving the problems - working round S0 - S3

Workaround for S3

17. The workaround for S3 is to remove the power cord for several seconds, then to replace it. Power-on the iMac in the usual way; it will usually then display one of the other symptoms.

Workaround for S0

18. If you are stuck at S0, then proceed as follows:

19. If you are still looking at the frozen grey screen, and after waiting for a few minutes the iMac has not responded, perform a hard power-off by holding the power button for 3 - 5 seconds. Then remove the power cable, disconnect all peripherals except the keyboard and mouse, wait several seconds, and replace the power cord.

20. Power-on the iMac (using the main power button is best). At the same time, try the following to eject the CD:

    • hold down the mouse button
    • if that doesn't work and the iMac boots up, you may be able to eject it in your usual way
    • if the iMac doesn't boot up then hard power off, remove the power cord for a few seconds (this may not always be necessary), replace the cord, and this time use an unfolded paperclip or similar to hold in the emergency eject button while you power-on the iMac. The eject button is actually located inside the CD slot, at the right-hand end. Feel for it with the paperclip. You should feel a slight click when you press it in. After the CD ejects you may need to power-off the iMac as this process disables the CD drive
    • another way of getting the CD out is to enter Open Firmware mode on startup. After powering-off the iMac and removing the power cord for a few seconds (where necessary), press the power button and immediately hold down the 4 keys <Apple> <Option> <o> <f>. This should get you into Open Firmware mode. Type "eject cd" (without the quotes) and press <Return>, then type "mac-boot" and <Return> to continue with a normal boot. While in Open Firmware mode, make a note of the Boot ROM version and date.

Workaround for S1

21. Symptom S1 (iMac boots with a blank/black screen) is usually worked round by using the "sleep/wake trick". After the iMac has booted up (wait for the sound of the HD activity to stop, then wait a bit longer), briefly press the main power button to send the iMac to sleep. The power button should pulse orange. Wait a couple of seconds more, then hit the spacebar to wake the iMac back up. You should now be presented with some sort of display, albeit probably rather dim and strangely coloured. This is in fact symptom S2.

22. Next time you reboot you may find that you have the dim display (as opposed to the blank display) immediately from bootup.

23. Another workaround for S1 is to use an external VGA monitor, typically borrowed from a PC. Nearly all slot-loading iMacs apart from the first (350 MHz) model have a small removable panel just above the memory access panel. Behind this is the VGA connector. Note that the iMac video output is at relatively high refresh rates and so older VGA monitors from PCs may not be able to display the screen image. Also some modern flat-panel monitors have been reported not to work.

Workaround for S2

24. You may be able to proceed with resolution of the problems even with the strangely coloured and dim display of symptom S2. Don't try to adjust the settings in the monitor control panel. Instead, darken the room you are working in.

25. Alternatively, use an external VGA monitor if possible as this tends to make working with the iMac much easier.

26. If you are unable to get any sort of visible display (e.g. because you have a 350 iMac and the sleep/wake trick is not working) then you may have to proceed blind. See section VI (para. 39 onwards) for suggestions.

V Resolving the problems - upgrading the firmware

27. If you can boot the iMac and get some sort of display, then you are ready to proceed.

28. To update the firmware you must boot OS 9.1 or 9.2.x from a local writeable drive (i.e. not from a CD). If one of these is already installed on your HD then proceed to para. 34. (Note that by "9.2.x" we mean 9.2 or any sub-version of 9.2.) It is quite possible to have different operating versions installed at the same time, e.g. OS 9.2.2 and OS X 10.2.8.

29. If OS 9.0.x is installed then an update to 9.1 can be downloaded direct from the Apple website: OS 9.1 update for iMac with OS 9.0.x already installed. It is a large download - 80MB - and will take a few hours on a dial-up connection. It may be more convenient to use a broadband connection and burn the updater to CD.

30. Follow the instructions carefully. In particular, to run the OS 9.1 updater you should first boot from you original OS 9 install CD by inserting the CD at startup and holding down the "c" key.

31. If OS 9 has been removed then reinstall from your original system CDs that came with the iMac.

32. Alternatively an OS 9.1 install CD can be purchased at relatively modest cost direct from Apple or other suppliers such as or Other World Computing (link to OS 9.1 item). This may be your only way forward if your existing system is OS 8.x or you have lost your original CDs.

33. If you have more than one OS version installed you can use the Startup control panel if necessary to select from the installed OS versions the one you want to boot (which must be 9.1 to 9.2.2 if you are going to update the firmware). Alternatively, hold down the Option (Alt) key on bootup and you should be presented with a list of the OS versions available to boot.

34. So, boot 9.1 or 9.2.x from a local drive, download the firmware 4.1.9 updater from the Apple website: Firmware 4.1.9 for slot-loading iMac and run it. Be sure to read and follow the instructions carefully.

35. Occasionally there are problems in updating the firmware. Possible causes/resolutions are: RAM not of suitable specification; minor directory damage on the disk - run disk first aid to clean the HD; USB or FireWire hardware attached to the computer - make sure all peripherals apart from keyboard and mouse are disconnected; a file or old firmware file is in the Startup Items folder and is causing a conflict - make sure that the Startup Items folder exists inside the System folder, and is completely empty (if necessary make a new folder called "Startup Items (disabled)" and temporarily move the items across); sometimes the updater has trouble if there are multiple OS systems on a single HD volume so make sure the 9.x system you are booted from to do the firmware update is the actual "system" selected as the boot volume in the startup disk control panel; if that doesn't help and you are getting desperate you could try removing the old system; you could also make sure you put the firmware updater on the desktop and run it from there.

36. During the update process, after the long tone described in the firmware update instructions the iMac should find the firmware image on the HD and write it to non-volatile memory on the logic board. This process takes a couple of minutes, depending on the computer speed. If you have some sort of video display you will see a progress bar across the bottom of the screen. At the end of the update the iMac will reboot. A PRAM reset is initiated automatically, but you may find that you need to do one by hand: shut the iMac down, power the iMac back on and immediately hold down the 4 keys: Apple - Option - P - R, and wait until the 2nd chime before releasing them. Your iMac is now fixed!

37. In the unlikely event that the firmware update is interrupted while the new Boot ROM code is being written to the non-volatile memory (e.g. if the power cord is removed) then the NV memory will be trashed and the logic board will need to be replaced.

38. If, when attempting to update the firmware, you hear a normal bootup chime shortly after the long beep, then the iMac could not find the new Boot ROM image for some reason, and it has proceeded instead with a conventional bootup. See para. 35 above for resolution suggestions.

VI Suggestions if you can't get a usable screen display

39. If your iMac is booting up but you can't get a screen display then you have a few options to consider:

40. Some people have managed to do a "blind" firmware update. First of all it will be necessary to update the iMac's OS to 9.1 (unless you already have this or a later version of OS 9 installed) and get the firmware updater onto the HD.

41. There are different ways of doing this - e.g. take the HD out and put it into another Mac (Donna's note- to avoid a common confusion, please be aware that firmware does not "reside" on the HD at all, even though it needs to be run from the HD. For example, once your firmware has been updated, you can put a new hard drive into that iMac without needing to update the firmware again. However, this drive swapping may be a way to get the updater onto the drive and then put the drive back into the iMac and try to install the update - if that makes any sense); or if file sharing is on then you might be able to access the iMac's HD remotely for updating the OS and copying the firmware updater over; or you could try starting the problematic iMac in "FireWire target disk" mode. This allows you to connect to another Mac using a FireWire cable and have the iMac's HD appear as an external disk on the other Mac, allowing you to install OS 9.1 and copy the firmware updater over. You may also find this article helpful: Macintosh: Computer Does Not Enter Into FireWire Target Disk Mode.

42. If you have access to software such as Timbuktu for remotely controlling another machine then this would be a convenient way of seeing the screen of and controlling the problematic iMac.

43. If you really have no way of getting a screen display, then try putting the unpacked firmware updater in the Startup Items folder (contrary to the advice above in para. 35). Then when you boot OS 9.1 (or higher), the updater will automatically start. You will need to dismiss 2 (?) dialogs by pressing <Return>: the first is a licence agreement, and the second is the dialog to shut the iMac down. You will have to use your skill and judgement in determining when the iMac is ready for you to press Enter each time! Once the iMac has shut itself down in preparation for the firmware update you can proceed as per the published instructions.

VII Tackling symptom S4

44. In symptom S4 the iMac thinks that there is a fault in the CRT or its circuit board (the PAV - Power/Analog Video - board) and shuts itself down. Your challenge is to get the iMac to start up so that you can update the firmware. Once you manage to get it going you need to avoid unnecessary restarts, and we advise you to plan (on paper) the sequence of steps you will take if you manage to get a successful boot.

45. The basic approach to overcoming S4 is to trick the iMac into starting up without it worrying about a perceived hardware problem. In this article we can only cover the simpler approaches. (WARNING: removing or replacing memory or other components potentially exposes you to high voltages, so make sure you carefully follow your iMac's instructions for the installation of these components. Also note that if you fail to take suitable anti-static measures then you risk damaging sensitive electronic components.) The following may help:

    • perseverance: you may find that occasionally, for no particular reason, the iMac will start;
    • moving/removing memory (and/or an Airport card) may do the trick;
    • or you might try removing all the memory, the Airport card (if present) and the logic board battery, and leaving for 24 hours. Then (in this order) replace the battery with a new one (or confirm good - 3.3 to 3.7 volts), leave for 10 seconds, reset the PMU, replace memory (one card only, in a different position from before), and plug in mains power. You are now ready to power-on the iMac using the button on the front. If you need to update the OS then boot from the original install CD by inserting it immediately on power-on and holding the "c" key. Otherwise boot from the HD as normal. If necessary, search the Apple Support pages or ask for help in the iMac(CRT) Usage discussion forum for more detailed instructions;
    • more advanced fixes exist but are beyond the scope of this article; check the iMac SL firmware problem solver site for details.

VIII Closing remarks

46. Resolving the boot/screen problems described is usually possible, though it can be a tedious process. For more help and advice, feel free to post a topic in the iMac(CRT) Usage discussion forum.

47. If you wish to retain OS 9 (as a bootable system or for Classic mode) we recommend upgrading to 9.2.2 as this is the most stable version. The updates are available on the Apple site: OS 9.2.1 (update from 9.1.x); OS 9.2.2 (update from 9.2.x).

48. This article has been compiled from the experiences of many users; thanks to all the unnamed users who have contributed the fixes.

Appendix: Useful startup tricks

These may help you diagnose and fix various problems with your iMac, including the firmware-related problems described here. Usually you should keep holding down the key or key combination until you see evidence that it has had an effect. Note that not all of these work on early firmware versions or on all early iMacs.

    • Extensions off: hold down <SHIFT> during startup to boot without extensions
    • System picker: hold down <Option> during power-on if you want to select which of the installed operating systems to boot
    • Rebuild desktop: hold down <Apple> <Option>
    • Eject CD: hold down mouse button during power-on
    • Boot OS X: hold down "x" to boot OS X (if installed)
    • Boot from CD: hold down "c"
    • PRAM reset: hold down <Option> <Apple> <p> <r> on startup and wait for the second chime before releasing. Warning! Do not try to reset the PRAM on a slot-loading iMac which has pre-4.1.9 firmware and has had an encounter with OS X, as this is likely to lead to symptom S4.
    • Open firmware mode: hold down <Option> <Apple> <o> <f> to enter open firmware mode on startup. This is the best way of finding out your firmware (Boot ROM) version. Make a note of the build version and date (4.1.9f1 BootROM Built on 09/14/01 is the latest). Type "mac-boot" without the quotes to continue with a normal bootup.
    • FireWire target disk mode: hold down "t"
    • Force power off: hold the main power button for a few seconds

Change history




23 Oct 2004


Minor corrections and additions throughout

19 Oct 2004


Completely re-written. References to tray-loading iMac removed as this model does not suffer from the same problems

Feb 2004


Original version