Adding Memory Issue

[Hoagie64]

Trying to install a second 8 gigs of RAM, and PC won't boot up. New memory stick works in place of the original stick. Both do not work together. 

[Prométhée]

Hello
if the module works alone, that it does not work with the original one, and that it works whatever the memory slot, then it is an incompatibility between two different modules, it happens!
An update of the bios, if there is neither, maybe nothing
I advise you to try with the same two modules
that said you would need the computer model to check if you can install more than 8gb!

Hello

this is a recorded response (macro))
we need the exact model of your computer, and the product number
To find it follow this
for a desktop computer

HP Desktop PCs - How Do I Find My Model Number or Product Number? | HP® Customer Support

and for a laptop:

HP Notebook PCs - How Do I Find My Product Name or Number? | HP® Customer Support

[Hoagie64]

[Hoagie64]

[Prométhée]

sorry but

Memory upgrade information

• Dual channel memory architecture
• Two 204-pin DDR3L SODIMM sockets
• Supports DDR3L SODIMMs:
  • PC3L-12800 (1600 MHz)
  • PC3L-10600 (1333 MHz)
• Supports up to 4 GB on 32-bit* systems
• Supports up to 8 GB on 64-bit systems

note:

* - 32-bit systems cannot address a full 4.0 GB of memory

HP Pavilion 400-434 Desktop PC Product Specifications

[Hoagie64]

Looks like photo download didn't work

HP Pavilion Slimline 400 PC Series

Product # J4W44AA#ABA

mODEL NO. 400-434

[Hoagie64]

I'm sorry, but what does your reply mean?? 

[Prométhée · Accepted solution]

What is it that you don't understand?
your motherboard is shown supported 8gb only
Sometimes you can install more, but not always!
If two modules are different, it often happens that they cannot work together

[Hoagie64]

I understand now. Thank you. 

[Prométhée]

Sorry, sometimes the translation is not what I meant