Chosen Solution
I have a Mid 2012 MBP, it has been working alright since 2012 but has gotten consistently slower so I replaced my hard drive to a SSD in summer 2018 and it was working fine on OS X Lion. When the Mojave update came up I decided to update it but unfortunately after the update my MacBook would freeze and shut down on multiple occasions and the error log would say kernel panics (similar to what was experienced before I changed my hard drive to SSD). On reboot, sometimes the flashing folder with the question mark or the no entry sign would appear; but if I leave my mac for a while and restart it later, it would work with no issues. Watching videos were an issue too as the video would freeze randomly and resume playing for a while. I reckon a clean install of the OS Mojave in recovery mode would help so I did exactly that. Tried a couple of times, the update keeps getting stuck at around 2 minutes left. I then tried command + R to install OS X- again, the update keeps getting stuck. I looked at the error logs, it says failed to verify installesd.dmg and it would download the chunklist again and loop through the whole process again. Going into to disk utility, my SSD was fine, but my original HD says SMART status failing. I erased my SSD, verified and repaired it just in case there were issues and tried installing the OS again but it just doesn’t work. I tried running AHT but it says error dec400, unable to run the AHT. Is this a hardware problem- HD cable problem? SATA cable problem? A RAM issue? Or is my SSD not working despite what disk utility says? Any help about how to resolve this would be greatly appreciated!!!
Was the ram upgraded on this MacBook? Try reverting to one stick (4GB) configuration if it has and try doing the install again. Last resort try creating a mojave install usb and use it using the steps here: https://www.imore.com/how-create-bootabl… This will require a working mac though otherwise you would need to use VMware (to boot Mac OS) on a windows computer to create a Mojave installer via USB.
I’m suspecting you have a few different issues here … First, your HD SATA cable needs to be replaced! The original cable is just not as good as the current one when supporting SATA III (6.0 Gb/s) SSD’s. Keep in mind your original HDD was only a SATA II (3.0 Gb/s) in addition if you had upgraded the HDD to a SATA III (6.0 Gb/s) the data flows of the SDD are more intense than the HDD which the older cable will struggle with. Here’s the part you need: MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable and here’s the IFIXIT guide MacBook Pro 13" Unibody Mid 2012 Hard Drive Cable Replacement. When you put in the new cable place a strip of electricians tape on the uppercase when the cable crosses over to help protect it from rough surface of the aluminum, you also want to not crease the folds! You want nice arcs as the sharp folds will damage the fragile foil wires and shielding inside. I also think you might have a RAM issue here. but in a slightly different way than what @benjamen50 is suspecting. Keep in mind when your system doesn’t have enough physical RAM for running what you are, your system will leverage the HDD/SSD as virtual RAM to make up the difference. So the failing cable can in fact also be the cause here. Now the last issue is how much free space your drive has. Smaller 128 & 256 GB need 1/3 of the drive set aside for the OS to use and larger 512 GB & 1 TB drives 1/4 and larger 2 TB & 4 TB SSD’s only need 1/8 th. So there is enough room for V-RAM, caching & paging files. I do recommend you up your RAM if you only have 4 GB and if you are running more intense apps look at going to 10 or 12 GB or just max your system at 16 GB.
The same problem ! Two of Macbook Mid 2012 ! AHT error DEC400 , hardware test failure ! Change HDD 3 times : 1 . original hdd 120 GB of my old Macbook , 2.hdd sata 2 = 320 gb 3.hdd original Macbook Mid 2012 =500 gb Reinstall Mac Os Sierra ot first Macbook ! Reinstall Mac OS High Sierra On second Macbook ! Remove second ram plate - DEC400 error I tried with RAM = 8gb Samsung (1600mhz) , 4GB Kingston (1600mhz), 2 Hiniks (1600mhz) -DEc400 error Change ram to 4 GB 1333 mhz - DEC400 eror Clean install mac OS Hight Sierra on USB Flash drive - DEC400 error Installing Mac OS X on Blank Hard Drive Using Internet Recovery - to Mac OS Mojave = DEC400 error Before these steps, I changed the platter RAM the first one Macbook to Samsung 2h8 GB low voltage 1,35v. After this , I set up RAM Sammsung on secong Macbook . It is possible that the error occurred before ! Pleace help !
Did anybody notice that the error screen mentions “Apple Diagnostics”. To me it seems to be an issue with the latest Boot Rom version 222.0.0.0.0. It tries to load Apple Diagnostics not Apple Hardware Test for some reason. And since Apple Diagnostics doesn’t work with mid-2012 it fails.
So it doesn’t matter how many reinstalls you do or which macOS version you use because once the Boot Rom is updated installing macOS doesn’t revert it back to an older version.
Hi, i have the same problem on my pro mid 2012 after some mounth of using mojave. It bagan freeze a lot and the same error dec400 when reboot. I changed memory but it was several years ago.. So i think if we all have the same mbp 2012 and mojave, maybe the problem is here.
