Chosen Solution

Running Apple Hardware Test turns up error message 4HDD/11/40000000:SATA(0,0) SATA cable (on your advice) and hard drive were replaced (with SSD) recently. Running Sierra. After running the AHT (associated with the fan running strongly and the strip behind the keyboard being hot) the computer then boots ok. This appeared to fix things once before, the other time I have no idea why it resumed working normally. When the device is down it will not run Disk Utility or safe boot. Running fsck did not help. Resetting NVRAM no help. Your assistance would again be much appreciated

mmm… The AHT error is one that is not well documented (outside of Apple). I’m thinking the logic board has an issue within the pathway to the Platform Controller Hub (PCH) and/or the PCH its self is having troubles. Basically, over the years the PC & Chip makers have reduced the number of chips it takes to build a computer. Very similar to how the old calculators had 12 or so chips. And now even the most advanced ones only have just one chip and its not even in a case just bonded directly to the PCB! Intel has done the same thing, taking about five chips and squeezing it down to one which we call the PCH chip. I don’t think swapping out the drive or cable will make a difference here. To see if only one SATA port is effected you could pop out the optical drive and plug the drive into its SATA port using a carrier like this: OWC Data Doubler. If that works it will only be a stopgap until you can either get the logic board fixed or replaced.

When it boots up hold command & R to boot into recovery mode. Open Disk Utility and reformat the drive you are using. Then quit Disk Utility and open Reinstall MacOS Sierra and follow the onscreen instructions. Also you can hold shift, control and option to reset the SMC.

Did you ever figure this out? Experiencing the same issue and also recently installed an SSD. I have a feeling it’s a heat issue, maybe from the SSD not being seated properly. Macs are finicky like that

Eventually decided it was the main logic board, and it all became too hard, so ditched it. I hope you have better luck