I have a HP ProLiant DL320 G5 Server that we have swapped out the motherboard with a working one, tried narrowing down removing different memory sticks, swapped out the onboard battery. I have a power supply tester that does not show a fault on the power supply. The fans kick on, no beep codes, no error LEDs turn on (or beep codes) with either the original or replacement motherboards. Anyone have any ideas?
Your Proliant DL320 G5 is fairly old and a very low-end system. Repair may not be worth it, given the options available now. But I'm assuming you're out of support and trying to self-repair.
This is a single power supply and single CPU socket server. That makes it easy to troubleshoot. I'd guess power supply, motherboard or CPU...
Your best indicator is the ILO2 log and the server's IML log. Have you looked into those yet?
I'd suggest following the troubleshooting steps outlined in the HP ProLiant Servers Troubleshooting Guide.
There are flowcharts which will help you narrow your issue down in a methodical fashion.