What's causing my iMacG5 to shut off unexpectedly?

My iMacG5 has been turning itself off without warning. It's as if someone just pulled the plug out of the wall. Four months ago I replaced the power supply, and the problem was solved -- until this week.

It started turning itself off again -- several times a day.

Is it really the power supply? Could it be something else? Is something else killing the power supply?

I've searched the web and the problem seems to occur often. But I've found no solutions. I've addressed power spikes & power dips, 3prong plugs & sockets vs. 2prongs. The computer is kept cool with external fans, and never gets very hot at all. So I don't think it's overheating.

Any ideas on what can be causing it and what can be done about it?
Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

sounds like a virus
Eoin OSullivanConsultantCommented:
Sudden power-down like you describe is most likely hardware related rather than a virus.

If it is an iMac G5 PPC it is clearly quite an old computer.  
What model?  
Are you running OSX 10.4?

You are correct that overheating or power supplies are a primary cause.

If you replaced the power supply, was it a new or 2nd hand part?  If it was not 100% new it could have failed as well.
sasnaktivAuthor Commented:
Nope! No virus.

And yes, I'm running 10.4.

I think the power supply was new, but hey, you never really know -- do you?

Is something killing the power supply? Or could it be something other than the power supply altogether?
Big Business Goals? Which KPIs Will Help You

The most successful MSPs rely on metrics – known as key performance indicators (KPIs) – for making informed decisions that help their businesses thrive, rather than just survive. This eBook provides an overview of the most important KPIs used by top MSPs.

Eoin OSullivanConsultantCommented:
I'd be tempted to think that the power supply has failed again as it is the most likely cause, it could be heating up and failing.
Can you contact the supplier of the power supply? Is it under warranty?
sasnaktivAuthor Commented:
It may still be under warranty. I don't know for sure. I'll check.
I concur with power supply as most likely. There is the strong possibility that there is something wrong with your logic board and a minute possibility that it is some other hw connected the logic board, like a peripheral, video card or hard drive. From what you have described, I would estimate the probability something like this:

Power Supply: 70%
Logic Board/Processor:29%

Isolate as much as you can, but it is probably worth the hassle of replacing the power supply even if that helps you figure out that you have a bad logic board. On those G5s, the processors were considered separate parts from the logic board. When they failed, you often needed to replace both. Tres expensive.

Your best fix, may be to buy another whole G5 if you really need the old processor or any intel iMac unless you have some crazy expansion cards for av or something like that.
Silly Rabbit, Viruses are for Windows
When was the last time you disassembled it and got out a shopvac and cleaned it thoroughly? Heat is probably the #1 enemy of older electronics, and nothing worse then dust/grime/dead bugs and such to facilitate this.

(Plus nothing to buy)
sasnaktivAuthor Commented:
The machine is clean & well kept. Heat's not the problem.

But there is something else wrong that I just noticed.
We have been starting it up using an external rather than the internal hard drive (I don't remember why, but it was necessary).

The internal HD was always mounted.

It no longer is.

Disk Utility doesn't even see the internal.
Shop Vacs can be very dangerous. It is a much bigger source of static than your synthetic rug. A lower powered vac would be advisable.

The mounting issue could be something simple like the Parition Map Scheme set wrong.

I am remembering now seeing two PowerMac G5's with the problem you are describing, they were bad logic boards, so perhaps it's 50/50 on the percentages listed above.
Well, I should have qualified that you don't use one of those giant static-electricity producing monster shopvacs, but thought that this was a given.  (Mine is about 10" high and 8" round and I have an extension that is non-conductive)

You can easily crash any O/S and have 100% guarantee of at least partial data loss if disks are getting disconnected without dismounting them.  Perhaps you are using the external HDD because the internal one has failed?  Sometimes these things are quite easy.  Run diagnostics and if it is bad, physically remove it.  Or, if the external one meets physical, electrical, and thermal qualifications, then just mount that internally.

By mounting it internally you eliminate points of failure plus overall power requirements, so you gain.
sasnaktivAuthor Commented:
Hey, how about a WetVac? Yeah! That will really put an end to this problem, wouldn't it?

I think my job for tomorrow is to pull out the internal hard drive and see what effect that has.
Who knows.

I'll let you know what happens.
Thanks for the attention,
Have you checked the console messages to see if there are any errors displayed just before shutdown?

I agree that the problem sounds like it might be heat related. There are various free utilities to track your CPU temperature and you might google for one to try.
sasnaktivAuthor Commented:
There's an external fan pointed at it at all times. The computer never really gets hot. It barely even gets warm. So that rules out heat as the cause.
That's good. Check the console logs.

Also have you run the hardware test that came with on disk with the computer?
No this does not rule out heat.  What if, for example a CPU fan failed. the Cpu would get hot quickly ,then clock rate would go down and maybe a core gets shut down or it goes into low power mode.   If that happens, then heat would always be low, because components got shut down so they are only drawing standby power.

You would still not feel hot air, but nevertheless, you experienced a thermal shutdown. There is no substitute for a physical inspection.
sasnaktivAuthor Commented:
Okay, So I checked out the error log, and it's meaningless to me.
I also tried to use the Apple repair disks as you suggested, but no matter what disk I put in, the computer just spits it out!
That's something new too.

Below is the error log:
mDNSResponder: ERROR: getOptRdata - unknown opt 4

Mac OS X Version 10.4.11 (Build 8S165)
2012-03-15 08:24:38 -0400
2012-03-15 08:24:39.800 loginwindow[73] FSResolveAliasWithMountFlags returned err = -35
Mar 15 08:24:40 MYsick-G5 /System/Library/Extensions/IOSerialFamily.kext/Contents/PlugIns/InternalModemSupport.kext/Contents/Resources/AppleModemOnHold.app/Contents/MacOS/AppleModemOnHold: MOHServiceAddedProc >>\n
LaunchServices: bad alias at 0x126257c, size 41083, CSStore size 959721779
LaunchServices: bad alias at 0x126257c, size 41083, CSStore size 959721779
LaunchServices: bad alias at 0x126257c, size 41083, CSStore size 959721779
LaunchServices: bad alias at 0x126257c, size 41083, CSStore size 959721779
2012-03-15 08:24:43.476 SystemUIServer[104] lang is:en
Main starting with pid 224 parent pid 61
2012-03-15 08:25:17.869 Skype[224] SkypeApplication::init called
2012-03-15 08:26:50.577 firefox-bin[228] GrP not requesting _registerDragTypes because the window is dying already
Mar 15 08:27:49 MYsick-G5 mDNSResponder: ERROR: getOptRdata - unknown opt 4
Mar 15 08:27:51 MYsick-G5 mDNSResponder: ERROR: getOptRdata - unknown opt 4
Mar 15 08:27:52 MYsick-G5 mDNSResponder: ERROR: getOptRdata - unknown opt 4
Mar 15 08:27:53 MYsick-G5 mDNSResponder: ERROR: getOptRdata - unknown opt 4
Mar 15 08:57:35 MYsick-G5 mDNSResponder: ERROR: getOptRdata - unknown opt 4

Open in new window

Nothing in this log of interest beyond a DNS config warning (network related, so no big deal)
Rejecting CDs + internal HDD failure + external drive + random crashes = intermittent internal ATA disk controller failure!
P.S. If it is internal failure, then disconnect internal power & signal to the CD & internal disk, use the disk you have externally and just hope it stops crashing.  Otherwise just throw the system away because it will cost less money to replace the broken one then it will be to get a new motherboard.   (Disconnecting power to internal drives serves dual purpose, as it decreases load on power supply)
sasnaktivAuthor Commented:
Tomorrow Guys!  
Guests are knocking at my door.
Thanks for the help, and have a good evening.
sasnaktivAuthor Commented:
Okay, so I pulled out the internal hard drive.
No luck!

This is the fourth time this iMac has had serious problems. Earlier it was covered under the extended warranty. This time it's not.

Actually all of our iMac have been in the shop for major repairs, and each of them, multiple times.

My advice to anyone buying any kind of Apple computer is to purchase the extended warranty, because 100% of ours have proven to be unreliable.

I don't know what more to do other than pull out the power supply and return it to the vendor since it's still under warranty. Trouble is I'll be without this computer until the issue is resolved.

Other than that, it may be time to replace it altogether.

If anything else strikes you guys please let me know.
You could ask for an "advanced replacement" which generally involves securing the replacement with a credit card, but not being charged. Then keeping your machine limping along while they send out the replacement.

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
sasnaktivAuthor Commented:
Thanks, I'll see if they let me get away with that.
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Apple Hardware

From novice to tech pro — start learning today.