Restarted and No Video Output - Only 2 Diagnostic LEDs

This iMac is getting up there in age, but so far it’s been very reliable, if not very slow due to the original HDD. I bought an SSD, temp sensor and all the works and was going to upgrade the Drive soon, but then I was greeted with this lovely issue one day out of the blue. I powered my computer off perfectly normally the night before. Then the next day I went to turn it on, and I heard the HDD and fans starting up, heard the Boot Chime, but the internal screen wouldn’t even try to turn on, no backlight (and no image holding a bright flashlight up).

I found a copy of the Apple Technician’s Guide online and was going through the flowcharts there. I ended up at the “No Video” Symptom Flow. I tested an external display, nothing; then I tried with known good RAM and still no luck. So following the guide, next I cracked it open and checked the Diagnostic LEDs. Only 2 were lit up after powering on. So next I replaced the PRAM battery, and then re-seated the GPU (with new Thermal Paste), and made sure there wasn’t a speck of dust inside anymore.

Then I connected everything needed to test the Diagnostic LEDs, and still only the first 2 were lit up, 3 and 4 were dark…

I saw other posts on here where some people found one of the Inductors on the Logic Board (L9000) was burned out, but I checked mine and it looks good still; I carefully checked most of both the Logic Board and the GPU and all the components look good, nothing burned out.

So at this point I am at a loss as to what could be causing this no video problem. The next steps in the Apple guide are to replace the GPU and then Logic Board if the problem persists, but I don’t want to dive that deep on just a hunch. Hoping someone here can shed some light on this for me.

Many thanks and appreciation in advance!

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@dmenc LED 4 will not light up because 3 is not. You have the manual so check these things to get passed #3

LED #1 + LED #2 + LED #3 = Power available, system is powered on, and video card found.

If third LED is not visible after power on:

• Verify that the MXM video card is seated properly

If POST boot chime is heard, go to No Video symptom flow. If LED #3 does not turn ON reseat video card and retest, then replace video card and repeat step 4. If issue persists, replace logic board.

L9000 usually will not cause a 3rd LED issues. Let us know what you find out.

@oldturkey03 Sorry if there's any confusion, but in my original post I mentioned that I was already following that exact flowchart, and I already have re-seated the Video Card and verified it and everything else is installed correctly. I'm just looking for some more insight before having to buy either a new GPU or Logic Board, I would rather not do that yet without knowing for sure what my issue might be. Also because I don't currently have a spare Video Card or Logic Board to test, I would have to find one for purchase.

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