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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.

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