Chosen Solution
I recently removed my iMac late 2015 i5 27” screen to upgrade the hard drive. (Something I’ve done successfully before). Upon re-assembly, the machine would not boot. Got a chime but then a full black screen. I removed the screen again, checked all of the connections, and they seem good. I get LED #1+#2 lit up on the diagnostic, but no #3. I’ve read that this means the the video card and logic board are not communicating. I’ve tried resetting the PRAM and the SMC with no luck. Questions:Are there any further suggestions about what to try to get it running again?Is there any value in pulling the whole thing apart to replace the PRAM battery? I read one place this could be a fix, but it seems like a lot of work if it’s a dead end. Thanks for any help. Update (03/16/2021) I took the entire iMac apart to replace the PRAM battery. This did not help, but it was a ton of work. Still getting only LED#1 and LED #2 lit up with a chime and a black screen. I DID plug into an external display that that WORKS. I put some fan monitoring software and got the following showing no fan sensor for the LCD Flying Lead.
I assume this is the thermal sensor monitor on the display. Either it’s damages or disconnected. Does anyone know if THIS is the sensor?
AND does it seem like this might be a solution to getting this machine up and running again. I don’t want to quit on it! Thanks Update (04/23/2021) I’m throwing out one last request for help with this machine. (Late 2015 iMac) I have scoured the forums here and elsewhere and have tried every single thing I’ve found short of replacing the logic board. Are there any other ideas before I have to strip this iMac down and sell it for parts? Summary of the problem: Symptoms: Screen stays blackFans go at full speedComputer boots and work with an external monitor, although throttled down due to fan issueFlying Lead Sensor still doesn’t show up in iStats. Repair attempts: Replaced LVDS CableReplace PRAM BatterReset PRAM and SMCReplaced screen with know working screen (symptoms were exactly the same)Replaced the thermal sensor cable on the back of the LCD panel.Fully removed all internals are replace to check every cable connection was firm.Ran Apple Hardware test got these results:There may be an issue with the System Management Controller (SMC). Reference Code: PFM006There may be an issue with the display. Reference Code: VFD001 Anybody here have any further ideas for a fix? Thank so much in advance.
@ericsoup So a quick search on this topic revealed this has been answered before. I recommend you do a search in the future to help you solve your issue faster. Hope it helps as it’s from a great source @danj iMac won`t power on. Power button pressed = LED # 2 lights up briefly
@ericsoup - Have you looked at is the eDP connector on the logic board and the eDP cable as you could have stressed them. And then you could have damaged the display its self in the process of taking it off not using the correct tools (a common issue for first timers)
it a corrupt SMC nothing can be done had the same problem replace the motherboard works fine now
