Help! I've installed a new PSU and now my PC won't boot.


Oct 3, 2014
Help! I've installed a new PSU and now my pc won't boot.

I recently decided to upgrade to SLI 970s, and purchased a corsair RM850 gold+ fully modular PSU to provide the increased oomph needed, replacing my old nzxt semi modular 650W PSU.

My motherboard is an asus z78-a.

I uninstalled the old gpu, put in the new guys, then got to work on the PSU. Firstly, I unplugged all the non-modular parts from the old PSU, namely the 24pin atx, the 4,4 cpu power connector and the 6,8 PCI power connector.

Next, I removed the old PSU, leaving in the system the modular parts, namely the sata/peripheral power connectors. I then installed the new PSU, with the atx connector, cup connector and 2 PCI connectors in place, ran them through the case, plugged them into their respective necessary locations, plugged the old modular sata/peripheral connectors into the correct location on the PSU, plugged the pc in (at which point all the mobo LEDs came on as expected) and attempted to boot.

The PSU clicked once, the case (not cpu or gpu) fans spun briefly then stopped, and my external hdds came on and started to spin.
Nothing unusual, I can't recall the last time I replaced a component and the pc booted first try. At first it looked like no power to cpu, which has happened in the past, but I've checked every cable and can't seem to find the problem.

I've tried unplugging the gpus, I've removed one (to access the CMOS reset), reset the CMOS, nothing has worked. Same result every time I try and boot.

Actually, I've just noticed that my external HDDs start spinning as soon as there's power to the mobo, not when I try and boot, but I'm writing this on a phone and it's a real pain to go up, so apologies for including this at the end.

Thanks in advance for any help.


You must use the cables that came with the new PSU. If the old ones are wired differently, you're probably introducing an immediate short-circuit to ground.

Another thing I'd do is perform your upgrades one at a time. Replace only the PSU (and its cables) first, with your old graphics card. If that works, replace the graphics card with a single GTX970. If that works, add the second one.


Mar 30, 2009

That old supply had more than sufficient oomph. If you did not damage something, then that original supply should boot the machine just fine.

Appreciate that most computers do not even consume 300 watts. But supplies must not be selected on watts; must be selected on current for each DC voltage. Most all computer assemblers would not know how to do that. So we make things simple. We tell computer assemblers to select a supply that is double what is required.

Unfortunately, resulting urban myths assume computers consume 600 and 900 watts. Is your computer so hot as to toast bread? Of course not. It does not consume anywhere near that power. Your old 600 watt supply should work just fine IF you have not created new damage.