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Hi I have an iMac 27 EMC2429 A1312 that all a sudden get a full white screen while surfing on the web (after 30min). I had to shut it down by holding the power supply button and it never started again :( I felt like 5 years ago when my previous Imac24-2009 get the video card burnt… I’m not a lucky boy ;( For my iMac 27, the symptoms are now: black screenno chimethe fans run slowlyLed1 is on and Led2 turns on when push the start button.nothing moreOf course I tried all the basic steps suggested like resetting the SMC, the VRAM … and all you can find on this website: no change I brought the machine to a repair shop center and 2 days later they told me it is the mother board. How to be sure ? If confirmed, should I also change the CPU itself ? and what about the graphic card ? It seems Leendert faced the same symptoms than me. For him the graphic card change was the answer (not the Mother bard) . So now I’m confused and I’d like to have you opinion before looking on eBay Note: My iMac has always been connected powered through an APC backups CS600, and I never shut it down in almost 4 years (or let say twice a year :). Note2: my iMac is unfortunately concerned by the VideoCard issue but had not been replaced by Apple (who extended exptionnaly the warranty for this series…but the previous owner of this machine didn’t d it) So far I never faced the video issues symptoms. Note3: I used smcfancontrol for 4 years that forces the fans at a higher speed than usual to be sure it will not be too hot (bad experience with my previous Imac24 where -also- my video card burnt) So now the question: changing the MB, the video, the PSU ? If someone can help :) Thanks Patrick

@macpat06 “2 days later they told me it is the mother board” it’s hard to say if it is or if it is not. After all you took it to the shop to diagnose it for you now you need to make a leap of faith. If they tested it and if it is a decent shop it probably is the board. Here is the thing that I would do. I would follow the troubleshooting guidelines for your model iMac and see what I could find out. After all, errors like your iMac’s can also be caused by bad memory, bad hdd/ssd and a couple of other things. Once you make the determination of what is going on with it, you will know what to repalce. Right now there is just still to much going on to say yes it is or it’s not the logic board. No-Power_No-Startup.pdf LED #1 • Indicates that the trickle voltage from power supply has been detected by main logic board. This LED will turn ON when you connect the iMac to a working AC power source. The LED will remain ON as long as the computer is ON or asleep. • When computer has been correctly shutdown, the LED#1 behavior may differ: • - If a startup event is scheduled in System Preferences/Energy saver, LED#1 will stay ON after a correct shutdown, • - If no startup event is scheduled in System Preferences/Energy saver, LED#1 will turn OFF and will stay OFF as long as power cord is kept connected and AC power source is present. Disconnecting the power cord and plugging it back will turn back this LED ON , even if computer is still off. • After disconnecting and reconnecting the AC power source, this LED could remain OFF if the AC power source is missing or disconnected , if the logic board is disconnected from the power supply or from the AC receptacle, or if the power supply board is faulty. LED #2 • Indicates that computer is turned on. This LED will be ON as long as computer is turned on (but is not asleep) and power supply and voltage regulators are working correctly. LED #3 • Indicates that computer and video card are communicating. This LED will be ON when computer is communicating properly with video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the backup battery (on back of logic board) may need to be reseated, or the video card might be installed incorrectly or needs replacement. LED #4 • Indicates that computer and LCD panel are communicating. This LED will be ON when computer is turned on and video signal is being generated. If LED#4 is ON and there is no image on display, then the LCD panel, the LED backlight board, or the cables between LCD and logic board or backlight board might be installed incorrectly, or need replacement. LED #1 + LED #2 = Power available, and system is powered on. If second LED is not visible when power button is pressed: • Verify power button connection to logic board • Verify power button functionality • Verify cable connection between power supply and logic board • Verify power supply • Verify logic board 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 • Verify if boot chime is present and fans are running when powered ON (reset SMC and PRAM, reseat or check backup battery if necessary for proper boot up): If POST boot chime is not heard, go to No Startup symptom flow, If POST boot chime is heard, go to No Video symptom flow. LED #1 + LED #2 + LED #3 + LED #4 = Power available, system is powered on, video card found, and internal LCD found. If fourth LED is not visible after power on: • Verify internal DisplayPort cable connections between LCD panel and logic board • Inspect LCD display cables for cable damage • Verify external video functionality, and according to result check the following items: If external display works then verify/replace the LED backlight board If external display works then verify/replace the LCD panel If external display does not work verify/replace the logic board