iMac Diagnostic LED mystery


New member
The customer's 2019 27" iMac reported to not boot up. Upon checking, the fans and HD spin up after power button is pressed, but nothing else happens. Backlight on screen doesn't even turn on. I removed the screen to get a better look and see what the diagnostic LED's had to say, and to my surprise, they lit up like so: Plugged in, system off: OFF ON OFF OFF OFF. According to the diagnostic data I can find, that doesn't make sense, as the 2nd LED is only supposed to light up after a successful boot, correct? And the first LED is supposed to stay lit as long as power is present and the power supply is providing adequate voltage. If I turn the computer on, they light up like so: OFF ON ON OFF ON. For kicks I replaced the power supply, but that changed nothing. Is this a good candidate to mail in for repair?


Staff member
First of all, wlecome to the forum!

Did you try to remove CMOS cell?
Also do a SMC reset.

What is 820-xxxx board code?