iMac a 1418 upgrade won't boot

I suggest going back to square one, remove all the upgrades, reinstall the original processor, and trying to get a working iMac.

Then do the easy upgrades first, then doing the subsequently harder ones.

If you had an model 14 iMac, your PCIe bus SSD might work. But you have a model 13 iMac, and it is a different type.

Look at OWC and you will see the difference...

Model 14, uses OWC Aura Pro X2 SSD's, iMac model 13, OWC Aura 6G SSD.....

What is the EMC of the IMac?

What processor did you remove, and what processor did you install?

There were two basic logic boards, the 65 watt one, and the 77 watt one.

You need to use a laptop processor, which has an "S" suffix, 65 watts,

not the desktop processor, which is 77 watts...

https://ark.intel.com/content/www/us/en/ark/products/codename/29902/ivy-bridge.html

https://en.wikipedia.org/wiki/IMac_(Intel-based)

What SSD did you install, and what is the maker of the memory?

Macs are extremely sensitive to the quality of RAM modules. Even if a module has the correct specifications it doesn't mean they will work. The only two vendors of RAM modules that have been proven over and over as totally reliable are Crucial.com and MacSales.com (OWC).

Remove all modules and replace them with just the Apple RAM modules that came with the Mac and see if it will boot. It is does then it's the modules that are incompatible. Get replacement modules from either of the two vendors previously mentioned.

I think you have the wrong PCI SSD...

My info says you need a Mini-SATA III, your trying to install a PCIe M.2...

Look on OWC for memory and slot SSD drives, and for SATA II SSD's (Pre 2010 IMacs, 3 gigabytes/second)

https://eshop.macsales.com/shop/ssd/owc/imac

For 2.5 and 3.5 platter and SSD drives, which are SATA III(3), 6 gigabytes/second, I use NewEgg...

Hey there thohahente,

Thanks for reaching out to Apple Support Communities. We understand your iMac will not start up as expected after some physical upgrades. We’ll be happy to help.

To verify, were the power supply board and logic board replacements done through an Apple Store or Authorized Service Provider? Did these issues occur immediately after these changes? If so, you’ll want to reach back out to where your Mac was serviced at and let them know. 

If you recently changed the RAM and these issues began, review the info from the following article to make sure you’re using the correct type of RAM with your Mac and that it is installed correctly:

Install memory in an iMac

We hope this helps. Take care.

You need to do upgrades one at a time.

First, do the memory.

Make sure it works, and do a 24-48 hour burn in...

Next, do the SSD. Put the new drive in an external enclosure, and install a fresh Operating System on it.

Once installed, boot from it, and make sure it functions. Then install it in your iMac... Do another 24-48 hour burn in...

Last, do the processor upgrade.

Did you mean "old processor 3330S"

and new processor 3770S?