yellow A B green C D on back of dimension 8400 how do i fix

[007zim]

yellow A B green C D on back of dimension 8400 how do i fix

 

[Gam3r01]

Here is the complete list on page 3 ftp://ftp.dell.com/Manuals/all-products/esuprt_desktop/esuprt_dimension_desktops/dimension-8400_Service%20Manual_en-us.pdf
Its a memory failure. It is detecting your ram but there is an issue.
From the site: If you have two or more memory modules installed, remove the modules, reinstall one module,
and then restart the computer. If the computer starts normally, reinstall an additional module.
Continue until you have identified a faulty module or reinstalled all modules without error.
l  If available, install properly working memory of the same type into your computer.