Give this a read: How to invoke and interpret the Apple hardware tests. While its a bit dated it still will give you a good idea on many conditions.
Lets tear into your 4SNS/1/C 0000008:TAOP--124 AHT error
4SNS - System sensor
T - Temp
A - Ambient air sensor
Given your systems slow down and the fan going full throttle this is an expected error. So it doesn't help localize the root issue.
Lets see if we can do some other diagnostics to help here. I would download this app: Temperature Gauge Pro. It can better identify what the different sensors are doing.
I suspect you have a known issue here which is the optical drives SATA port is not able to support your HD SATA speed. Review this: OWC Data Doubler. Note at the bottom of the pages (you'll need to scroll down) in Red the following:
Testing has demonstrated that Apple factory hardware does not reliably support a 6G (6Gb/s) Solid State Drive or Hard Disk Drive in the optical bay of 2011 MacBook Pros (ModeI ID8,1; 8,2; 8,3). If your OWC Data Doubler bundle comes with a 6G drive, you should ONLY install that drive in the main drive bay and utilize the Data Doubler to re-task your existing drive or install a new 3G SSD or HDD in the optical bay. PRE-2011 models can utilize a 6G drive in the optical bay, but will do so at a reduced 3G (3Gb/s) speed.
While you do have a '12 model the issue is still present and it makes no difference who's HD carrier you use as the issue is within the systems controller chip not the carrier.
OK, how can you prove this?
The thermal sensors should show the CPU & HD are running hot. Using Apples Activity Monitor You should see the indexer process is using a lot of CPU cycles with most everything else almost no cycles.
What is happening is the system is unable to read/write data to the HD reliability. If you kill the indexer process your system should cool down as long as the process is stopped. This is the symptom of a mis-matched SATA I/O channel. You'll need to look into getting a slower SATA spec'ed drive: SATA III (3.0 Gb/s) Vs the SATA III (6.0 Gb/s) drive you currently have (the original Apple drive).
You do want the process to run so you can't kill it as your file system requires it. You do need to address the I/O channel issue.
You can place the HD externally using a USB or Thunderbolt connected case to house the drive as a short term solution.