Added GPU to computer and it turns off after a few seconds?

Feb 15, 2018
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I recently purchased a GTX 660 for my old Dell Computer to revitalize it but the minute I put it in and added the 6pin connector required for it the computer will not start.

I have a off brand 550W powersupply and I used it without the 660 initially and it started up just fine but when I install the 660 and connected the 6pin pci-e connector to it it simply started up at the windows 10 loading screen and then the whole pc shut off. I had an old AMD card in there before and I don't know if I properly uninstalled the drivers so that might be the reason or it might be the PSU.

Here's the specs:
i5-2400
8GB DDR3 Ram
550W (RetailPlus Powersupply I have a feeling it's this but not sure)
Asus GTX 660 2GB


If anyone could help me troubleshoot the issue It'd be appreciate thanks!