Please check if you have the ECC or Non-ECC memory and DO NOT mix ECC with Non-ECC memory.
This system require to install memory in pairs
Your Apple Power-Mac G5 Dual-Core 2.5GHz (M9592LL/A) only supports memory modules made with a specific type of chip. Should you find what seems to be the exact same memory modules elsewhere for a lower price, it is very possible that the cheaper memory modules will not work in your system. By using Memory-Up's Memory Finder, your selected memory modules are Guaranteed To Work or your money back.<Learn More>
Determine which type of user you are and how much memory do you currently have first before buying the new memory modules.<Learn More>
Different sizes/speeds of memory module can be used together as long as the specifications meet the system requirements.
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Apple Power-Mac G5 Dual-Core 2.5GHz (M9592LL/A) Specification