I get a white screen upon power on.

Apple iMac "Core 2 Duo" 2.4 20-Inch (Early 2008) Specs

Identifiers: Early 2008 - MB323LL/A - iMac8,1 - A1224 - 2210

I'll change the iMac icon to the correct one. Hook it up to an external monitor and see if you get the desktop. It sounds like it may be hanging on boot up. Try holding down the Shift key on startup to try starting in Safe Mode. Try what remac said, select the drive on the left and run the repair in Disk Utilities till it runs clear.

Start it from a installation DVD. Hold down the "c" key with the disk insertet. If it starts this way go to the second screen (the one after the language choice) and select Utilities -> Disk Utility.

Verify the soundness of the HDD.

Please post the results.

Really late to the party but I'd like to point out somethings.

To reset the parameter ram on a Mac you hold down cmd+opt+p+r and not control it has been this four finger salute since the compact Macs.

White screen can be a number of things so before you tear down your iMac which is a real PITA to do you should determine if it is something simple. Did the pram reset work?

If not and you have an external monitor handy and the Apple external monitor adaptor try plugging it in and see if you now have a desktop. If yes it might be an idea to backup or copy off any files and data that you might want should things go bad further on down the line.

If you did not get a working display on the external monitor you could try running an install DVD if you iMac came with one or if you can get a hold of a working iMac of the same generation you could try building a USB boot key by cloning the known good OS onto the USB key and booting your Mac with it. I recommend using Carbon Copy Cloner available here, https://bombich.com for this task if you build USB key. Typically though if the HD is bad or the OS is corrupted you will get an error on screen.

If you do see your desktop on the external monitor, go to System Preferences -> Displays and check to make sure that your primary monitor is selected. This is done by dragging the bar from the external display (2) to the primary display (1). Did this suddenly solve the problem?

If not it may be something a bit more involved. You could try reinstalling your operating system though it's also possible that your Mac has succumbed to a known failure issue which is a bad video card. Some folks have reported success with reheating the pins on the failed card using a mini butane torch. I have used this method on some Macs and PCs over the years and though it does initially work there is the chance that at some point the card will fail again. However if your iMac falls into this category and Apple will not help you out with their hidden extended warranties then this might be your only option if you feel adventuresome. Btw there are some who put boards in toaster ovens or full size ovens, personally I would not do this and if you went to a proper repair shop their will have an infrared rework station that is safer than using the micro torch method but also a more expensive option.

At this point though you are talking about getting really down and dirty with the insides of your iMac and this might be exceeding your comfort level. So if you think your iMac is scrap you might want to consider removing the drive and recovering what you can if the external monitor trick did not work. There are plenty of inexpensive external SATA hard drive cases or even SATA docks on the market that you can plug your hard drive into which can then be opened on another working Mac. Also a number of Youtube videos on the subject, you will need a set of small Torx screwdrivers as Apple tends to use these and Phillips on their computers.

I currently have an iMac A1224 on the bench with white screen, replaced the ram, reset PRAM as well as did a full reset (shift+command+p+r) and hooked up an external monitor. The external monitor shows the desktop fine and the Mac functions in this manner however the built in screen is still white which makes me think in this instance that the screen itself has failed.

Credit card in the superdrive will get the disk out. I have the same problem with my 20" 2007 iMac. Chime, white screen, all drives spinning and powered, then nothing. The only keyboard shortcut that appears to be working is PRAM reset. I tried all of the above tricks and every other one I have been able to find. External monitors, known good RAM, new drive, new disks, 3 keyboards, target mode, and many, many more.

I finally gave up and took it to the Apple store. Three weeks later they told me it was the logic board. $435. I called Apple, emailed Tim Cook, dealt with Apple executive relations. They finally waived the labor at the store - $39 discount. woo-hoo. They refused to tell me the failure rate for this component or what rate qualifies as a known issue.

I'm looking for a "soldering iron" type solution now. I refuse to pay Apple's part-swappers $400 for what is probably a $2 capacitor fix.

My iMac early 2008 started to boot to a white screen. Replaced the logic board, no change. Removed the graphic card, gently set it up in a vice and applied hot blown air with a paint gun for about 3 minutes to reseal any broken solder joints. Let it cool and reinstalled with new thermal paste. Problem cured.

Always, always, have a USB boot disk to hand. It makes diagnosing problems much easier.

From my experience, a dead hard drive will result in a question mark being displayed on boot. Faulty RAM will involve a black screen and beeps. A failed display will give you the chimes but a black screen. On the logic board, there is an array of LEDs below the display connector, which will light up if the board is operational.

The white screen boot problem sounds like a bad graphics card. From my experience, the GPU is the most likely thing to overheat on an iMac. As you cannot easily open the machine, dust accumulates inside it, trapping airflow and eventually leading to overheating failure. Typical symptoms are random lines appearing across the display at all times, and / or booting to a blank screen. However, you can still usually boot to safe mode, even when the display is messed up. If you’re really stuck, you can hold down Control+S on startup and go into single user mode. If that works, your CPU, disk and RAM is probably okay - or at least okay enough to get a kernel initialised. You can then recover data from the disk.

Resealing the old card’s joints with a heat gun sometimes works, as does finding a replacement GPU from a donor Mac. For this model, it’s an ATI Radeon 2600XT. If you’ve already done a hard drive upgrade on a pre-2012 iMac, and fitted a CPU with thermal paste before, you should be able to swap out a graphics card in under two hours.

It’s somewhat ironic that given their reputation as “sealed” units, these old iMacs are some of the most upgradable!