One of the red leds for dimm 3 raiser b lit up red on my MacPro early 2008 lit up, so I reseated the ram in question on it by switching it with it's counter part (swapped dimm 3 for dimm 4) and switched it from the top to the bottom riser. Then I had the full amount of RAM again and no red light.

I also run Ubuntu 16.04 LTS on it.

I have never reset the nvram, pram, or SMC on it, and so I was wondering if that is somehow related to such strange events in the hardware as the led lights coming on and going away after reseating the ram as I did.

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    Just to clarify a couple of things: (1) Did you experience any symptoms, or was it just a coincidence that you happened to notice the red light? (2) Are you saying you've never reset the NVRAM or SMC at all, or that you'd never reset them until now? – Monomeeth Aug 27 '18 at 6:41
  • (1) Symptoms such as...restarting? Not too much...it's happened but not very often...there's 24GB of RAM. (2) I've never reset them. – leeand00 Aug 27 '18 at 12:56
  • Ok, I guess I was asking because the implication behind your question is that you've reset the NVRAM and/or SMC and then you had a red light appear? But, if you've never reset them, then it's not clear why you're asking about resetting the NVRAM or SMC, unless you're really asking if you should reset them? Regardless, I've added an answer, so hopefully that helps. If I am missing something, please feel free to add a comment. – Monomeeth Aug 27 '18 at 22:47

As you may know, the Non-Volatile RAM (NVRAM) and System Management Controller (SMC) have totally different purposes.

In terms of the red light you're referring to:

  • resetting the NVRAM will have absolutely no bearing on this whatsoever
  • resetting the SMC could have an influence, but probably not in the way you're asking about.

At its core the SMC is responsible for power management and controlling the thermal environment of your Mac. Typically this covers things like interpreting and responding to various sensors, battery charging, sleep/wake/hibernation, the power button and restarts/shutdowns, LED indicators, and so on.

Since the red light needs a power source, as do RAM chips to operate, it's quite possible that resetting the SMC could have an impact on the light:

  • directly in terms of its own power source
  • indirectly because the associated RAM isn't getting the power it needs and therefore triggers ECC errors which triggers the red light.

However, despite the above explanation, in most cases a red light indicates nothing more than the fact the RAM isn't seated properly. Removing and reseating the riser(s) will usually rectify this problem. While it may seem unlikely that this was the problem in your case, it actually doesn't take much for this to occur (e.g. the computer is moved, bumped, etc).

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