Chosen Solution
My 2007 iMac recently started to reboot itself in the middle of the night. I turned it off, and in the morning it would not start- no chime, lights, nothing. I replaced the PSU. On first attempt it still wouldn’t boot, but after a few failed boot cycles it lurched back to life. All good for a few days. Then the random night time restarts began again. Now on startup, first 3diag leds light up momentarily (as does USB bus) but not 4th, fan spins momentarily then nothing. This repeats at 5sec intervals until power is disconnected. checked power cord reset smc haven’t been able to reset nvram tried different hard drives/no hard drive Any thoughts? TIA -Dave
Please give us the EMC # from the bottom of your stand so we can get you to the correct guide for replace or reworking the GPU. UPDATE This answer is not a firm solution. I’m just thinking it through. For the restarts to repeat, I would assume you have a component that is overheating and causing a shut down. Then it may try restarting on its own or it cantata a while to cool off and then be a blue to reboot. The there diagnostics light show that it is getting to the GPU (video card) and stopping. I have repaired several of these by doing a GPU reflow (which works about 70% of the time in my experience. iMac Intel 20" EMC 2133 and 2210 Video Card Replacement If you have access to an external monitor it might give us more clues. (If the external works but the display does not, go for the inverter board). iMac Intel 20" EMC 2133 and 2210 Inverter Replacement What I have seen fail on this models are the following in order: Partial Power supply failure Apple part # 661-4433 GPU failure Apple part6 #s 661-4436, 661-4440 Inverter board Apple part# 922-8204
