Thoughtful, detailed coverage of the Mac, iPhone, and iPad, plus the TidBITS Content Network for Apple consultants.

Solve Mac Startup Problems by Reseating RAM

In the normal course of affairs, my Mac Pro sleeps automatically at night, and I wake it up to work in the morning. Sleep, wake, sleep, wake — it’s a routine. So you can imagine my surprise the other day when I touched the keyboard to wake it up, only to be ignored. I mean, it’s one thing when I go in to wake Tristan up and he pretends to be fast asleep or is merely being a groggy pre-teen, but he always makes it out of bed eventually.

This time the Mac Pro simply wasn’t rousing itself from its electronic somnolence, even though I could hear its fan running. I figured it had somehow crashed, so I pressed the power button for a few seconds and it shut off. Pressed the power button again, and its fans started, but nothing more happened. No startup sound, no monitor activity. Actually, that wasn’t quite true. Instead of providing steady illumination, the power LED was flashing, something I’d never seen before.

A quick Google search via my MacBook on “Mac Pro power LED flashing” turned up a number of threads in various discussion forums, all suggesting that the RAM needed to be reseated. One of the discussions pointed at an Apple support article that gave a little more detail about the Power On Self Test (POST) codes that the Mac communicates via a combination of tones and blinking lights. It agreed that the first thing to try was reseating the RAM, and if that failed, to try replacing the RAM.

With my Mac Pro (Early 2008), RAM is installed in pairs of DIMMs on a pair of memory cards in a rather complex pattern. Since I have 10 GB of RAM, in the form of two 1 GB DIMMs and four 2 GB DIMMs, there are six DIMMs in play. Initially, I simply popped the memory cards out and slid them back in. No improvement. Next I took out both memory cards and ejected each DIMM, looked at it, and pushed it back into place. Again, when I reassembled the Mac, it wouldn’t start. (By unplugging the power cord on the Mac Pro for more than 15 seconds, I also reset the System Management Controller (SMC), which is also a good thing to do. But it made no difference, and I couldn’t zap the PRAM because the Mac wouldn’t boot at all.)

I was starting to get worried, but I had to go off to an appointment, so I left the Mac turned off, figuring that would also eliminate heat from the equation. When I returned, a quick test showed that heat wasn’t related, since the Mac still wouldn’t boot. I was starting to become concerned that a DIMM had gone bad, which was going to be an annoyance even if OWC was good about replacing it, as I expected they would be. The whole situation seemed odd, since the memory has been in this Mac Pro without being touched for over two years (and bad memory usually shows itself immediately). But reseating seemed unlikely as well, since the Mac Pro itself hadn’t been exposed to vibration or even opened in months, since I’d installed an SSD at the end of 2010.

Next I resigned myself to figuring out which DIMM was bad, to which end I pulled pairs of DIMMs until the Mac booted — I gave a major sigh of relief when I heard the familiar tone! — and then I started swapping individual DIMMs until I thought I had isolated the troublesome DIMM. But then I did something that’s always worthwhile when troubleshooting — I attempted to confirm that the problematic DIMM was indeed bad by installing it in a different configuration (in this case, on the other memory card). And you know what? It worked! I’d spent enough time fussing, so I closed up the Mac Pro and went back to work.

Nonetheless, I was worried that maybe the error was intermittent, so the next weekend, I rebooted the Mac with as little as possible running and then used the free Rember utility (which is itself a front end to the Memtest OSX command-line utility) to run memory tests on all the available free memory for 48 hours. The memory passed with flying colors, eliminating any concern about it. It’s still conceivable that a particular slot on one of the Mac Pro’s memory cards is bad, and several people on Twitter suggested using a contact cleaner on the DIMMs if the POST failed again. But with over a week with no trouble, I assumed that my reseating efforts solved the problem.

Celebration turned out to be premature, and a few days after I initially published this article, my Mac Pro again showed the flashing power LED instead of waking up. This time I quickly isolated the DIMM that was causing the problem, but to get it to work, I had to clean its contacts with my DeoxIT D100P pen before reinstalling it on the Mac Pro’s memory card. I did the same with the other DIMMs, and the Mac has been running fine for a few days.

The moral of the story is twofold. First, if your Mac’s power light is flashing, reseating the RAM is the likely fix. And second, properly reseating the memory may require a little more effort than simply removing and reinstalling the DIMMs or other memory modules.


Try productivity tools from Smile that will make your job easier!
PDFpen: PDF toolkit for busy pros on Mac, iPhone, and iPad.
TextExpander: Your shortcut to accurate writing on Mac, Windows,
and iOS. Free trials and friendly support. <>

Comments about Solve Mac Startup Problems by Reseating RAM
(Comments are closed.)

Adam, during your process it sounds like you likely reset the SMC on your Mac Pro. Could this have contributed to your success?

I also have a memory related issue with an early 2008 Mac Pro. My Mac Pro has 24GB RAM installed, but recently I noticed only 20GB is being recognized. Two 2GB sticks have just "disappeared". I've had no boot issues and was given no warning that memory failed, etc. Like yours, my computer sits in one place and is not expanded or updated (physically) often. Fortunately, I just received an expansion card, so I have a reason to open it up. I'm going to try reseating the RAM also. Thanks for the inspiration on a possible fix!
Adam Engst  An apple icon for a TidBITS Staffer 2011-05-13 13:07
Resetting the SMC requires only unplugging it for 15 seconds, so that happened as soon as I opened it up, and didn't make a difference. But thanks for reminding me, since it was something I checked and verified, but had forgotten to add to the article.

I'd definitely test all your DIMMs, in different configurations - hopefully that will clarify the situation.
Garner Lewis  2011-05-17 00:55
Reading this article is spooky! I had exactly the same problem on May 13 with exactly the same Mac Pro! A technician named Steve, through Apple Express Lane, worked with me to find the problem. I went through the similar procedures for removing and reseating. First I removed the top card and rebooted - no change. Next I switched and removed the lower card and the Mac Pro booted. I then removed two DIMMS then one. I then switched what I thought was the bad DIMM in another slot and it worked! What the ----? I tried various combinations. Sometimes it booted; sometimes not. Then I concentrated on the lower card and how it was seated. I discovered that the lower card had somehow wiggled its way loose ( about a 1/32 of an inch) causing the flashing power light. I cleaned the gold connectors; reseated the card and voilå. The system profiler says all the DIMMS are okay and it has been working perfectly now for 3 days.
John E. Payne  2011-05-18 10:55
Thanks for a useful article. BTW, a flashing power light is not necessarily a RAM problem. I have an old PowerMac G4 MDD, and its power light flashes a code (short-short-long) to show a backlighting problem with the attached Apple Studio Monitor display.