OptiPlex 745 not turning on

Collecting and reading the PDF manuals is a good place to start.

Most people throw them away because they have a hardware failure.

Hi Owen, 

A solid green power led and quads 3 and 4 lit indicate that memory modules are detected, but failing. That could indicate a problem with either the memory module(s) or the memory slots themselves.

Here is a link to the service manual - On page 181 it gives the diagnostics pattern codes and the link to how to remove memory.

The best way to troubleshoot this issue is to remove all memory modules from the system and then power on the system and then add each memory module in turn to determine whether you have a memory module fault or memory slot error. With all memory modules removed, power on the system. All going well the system should now have the number 1 led lighting up to indicate no memory is detected. Turn the system back off and insert your first memory module into slot 1 of the motherboard and then power it back on. If the memory module is working the system should then proceed to boot to the operating system. If the original memory error returns, take that same module and try it in the second slot. If it fails in the second slot that could suggest the memory is faulty.

If you have a second memory module, repeat the process starting in the first memory slot to see if the system then boots to the operating system. If at this point you get the same original error with 2 different memory modules in 2 different slots, this would indicate it's the memory slot at fault and the system would require a replacement motherboard. 

Alan