O oh! Kernel panic. Is it RAM? GPU? What next?

When the Mac Pro boots to the desktop, it's only there a few seconds before it freezes. So I don't have time to get into the System Profiler.

Some interesting stuff:

I put the broken machine into Firewire Target mode, and used another Mac to read its hard drive. The broken machine crashed after a few seconds.

So I tried it again in Target mode. But this time I removed the broken Mac's nVidea graphics card. It then stayed afloat for about 8 minutes before stopping. That was enough time to rescue a few of my files.

That makes me think the problem may be with the graphics card.

There is a known problem where Mac Pros can freeze when in target mode for a while, so that might have caused target mode to end after 8 minutes.

I don't think I can afford an Apple Center repair, so I'm thinking of ordering a graphics card to see if it makes any difference. Otherwise, I assume the next suspect might be the logic board, but I don't really know. I dread to know how difficult a logic board might be to install.

Thanks Grant, but I could not find any LED indicators near that RAM cards.

Some new info:

I finally managed to get the machine to do a short Apple Hardware Test without crashing. It can't do the long test because it always crashes. The short test gave this error message:

Alert! Apple Hardware Test has detected an error.

4SNS/1/40000000: TNOD- 127.875

Pass Number: 1,

Total Time Testing 47 secs

According to a CNET article that explains error code, this message means that the heat sensor that monitors the Northbridge chip on the main logic board is showing it is too hot.

Someone else posted a picture of the offending heatsink and sensor in an older forum. Apparently these sensors come loose and can be fixed with glue, but if you take it to an Apple Centre they only replace the entire logic board.

I've replaced RAM, graphics cards, optical and hard drives, but tinkering with the logic board is something above what I've done before. I'm a novice, but then again, I guess I've got nothing to lose. I really need my Mac Pro to work.

I think I will try to unscrew and inspect the Northbridge heatsink, and failing that I might order a new logic board on the internet. The thought of installing a new logic board scares me, but I don't think I can afford to pay an Apple service centre $1000+ to do it.

I fixed it!

Below I will show what I did, for anyone searching similar problems in the future.

The problem was the heatsink that sits on top of the Northbridge processor. It is located on the main processor board. See my photo below.

This heatsink became slightly loose because two plastic rivet screws that hold it down had become worn.

I bought some 3mm wide / 12mm long nylon screws from my local electronic store to replace the originals. The new screws only just fitted. 10mm long would have been better.

I had to remove the main processor board to do this. Something I'd never done before. To remove the processor board I had to remove both the main processors and heatsinks. But this was also a good opportunity to clean dust away from hard to reach places, using a new paintbrush and a can of compressed air.

To reinstall the processors, it required cleaning with isopropyl wipes and reapplying the thermal paste. I used Arctic Silver 5 brand paste.

It was a similar process for the offending Northbridge heatsink. I thoroughly cleaned it. I made sure the little heat sensor underneath was properly positioned, and used some glue to hold it there. I also applied new thermal paste to the top of the Northbridge processor, before attaching its heatsink with new nylon screws.

I had nothing to lose by doing all this myself. The other alternative would have been sending it to the Apple store to get a new processor board.

I was thrilled when the machine booted up and was running properly again. I also feel a bit more confident with computers now!

Thanks A Brody,

I don't have that much knowledge.

I just read other forums about how to remove processors.

The other incentive was that the logic board (processor board) would have been written off anyway if I had a professional repair done. They never fix things at a component level.

Having done it once, I feel confident if I have to remove processors and heatsinks in the future.

If you have an out-of-warranty machine, it's a good place to learn these things.