Imac 2011 hardware test error

That old Apple Hardware Test code generally indicates a bad or improperly seated RAM module.

see > https://www.techrepublic.com/article/how-to-decipher-error-codes-in-apple-hardware-test-or-apple-diagnostics/

or > https://logi.wiki/index.php/Apple_Hardware_Test_(AHT)_and_Apple_Diagnostics_(AD)_Codes

for RAM spec's and instructions, see > Install memory in an iMac - Apple Support

+1 it is likely either:

• Failed RAM
• Incorrect RAM
• Unseated RAM