What is this Diagnostic Error Code

That is a memory error.

Your 2011 MacBook Pro has replaceable DIMMs and can accept up to an 8GB DIMM in each of its two sockets, for a total of 16GB. DIMMs can be replaced by the each.

But the EXACT correct spec DIMMs must be installed. Too slow will not work at all, and too fast will be flaky.

Hello FKJ8,

Thanks for visiting Apple Support Communities. We understand that you're having an issue installing a fresh version of macOS on your MacBook Pro and that you're seeing an alert during the Apple Hardware Test. That alert could possibly mean there is an issue with the memory. Just in case, here's a guide on removing/installing memory in your Mac: MacBook Pro: How to remove or install memory

If service is needed, be sure to check out the information in the following guide due to the age of your Mac: Obtaining service for your Apple product after an expired warranty

Thanks again, and take care.

Many times the memory slot nearest the Bottom Case tends to fail due to cracked solder joints.

Is the original Apple memory installed or does the laptop have a third party memory upgrade? Macs are very picky about the memory they use so some third party memory modules may cause issues.

You will need to figure out if one of the memory slots is bad or if one of the memory modules is bad. It is very rare to have both memory modules fail.

It is also possible the memory has become unlatched. Make sure both memory modules are properly latched in place. Since a single shared cheap plastic arm is used to latch both modules, many times the memory nearest the Keyboard will become unlatched when inserting the second memory module.

FKJ8 wrote:

I transferred two of 4gb PNY total 8gb 1066mhz SODIMM DR3 Rams from MacBook Pro 2010 Core Duo to this MacBook Pro 2012 i5 which originally had two of 2gb Samsung total 4gb1600mhz Rams.

The speed of the PNY RAM was too slow for the newer laptop. Sometimes you can go the other way and use faster memory, but only if the memory manufacturer such as Crucial sells the faster memory for that specific system. It is very important to know the system requirements when making both hardware & software changes.

Glad the new Crucial memory is working.

If your computer was operating correctly using 1066 speed DIMMs, then 1600 speed DIMMs are NOT the correct DIMMs. I expect you will find that your Mac crashes randomly, on average about once a day, for reasons that do not have any pattern to them.

You MUST use the EXACT correct spec DIMMs in your very fussy MacBook Pro. Too slow will not work at all, and too fast will be flaky.

Grant Bennet-Alder wrote:

If your computer was operating correctly using 1066 speed DIMMs, then 1600 speed DIMMs are NOT the correct DIMMs.

Actually I was wrong in my previous reply. I thought the OP moved the memory to a 2012 laptop when in fact the OP mentioned a 2011 model (assuming this label is correct). The 2011 models used memory with a speed of 1333MHz and Crucial's website does not sell/advertise the 1600MHz RAM for the 2011 models (both early & late).

https://www.crucial.com/compatible-upgrade-for/apple/macbook-pro-(13-inch--late-2011)

https://www.crucial.com/compatible-upgrade-for/apple/macbook-pro-(13-inch--early-2011)

Maybe the OP is incorrect on the exact model of the laptop and it is a 2012 model.

All above is true, but if you are trying to install Monterey, you cannot do it because its not supported on a 2011 mac.