late 2009 21" iMac screen flickering

[mudpup]

My late 2009 21" iMac recently developed a screen flickering issue. The screen does not flicker constantly, just now and then, but it flickers several times every day. When it flickers, the light level sometimes decreases for a while. This flickering has occurred within the first few minutes of use as well as after many hours of use. I have installed all updates as they are released. I know the 27" iMacs of the same era had this flickering problem and that Apple issued a firmware update for those iMacs. Does anyone else with a 21" iMac have this problem? Does anyone have a fix?

 

[Robster3]

I am having same problem. Now we can only have it on low dim or it blacks out.

Any ideas?

 

[eduardrw]

I had the same problem on my late 2009 27".
It is either the display or the graphics card.

If you can connect an external monitor to see if that works fine - if so, its the display -> expensive on my 27"
If the external monitor shows the same problem its the graphics chip - means replacing the motherboard.

 

[mudpup]

update

Thanks, eduardw, I did not try this experiment because I don't have a convenient external monitor and the flickering only happened when it wanted to. I couldn't make it flicker. I think it is an excellent suggestion though.

I can't say this has been resolved but I have an update. The flickering on my iMac became so frequent and annoying that I was getting ready to drive over two hours to the closest genius bar for help. Then the flickering almost completely stopped. My leading theory is that summer got so hot that I have had the air conditioner running constantly and that the flickering was somehow caused by the increasing heat and/or humidity in my home before I gave in to the air conditioner. Of course, that could be a coincidence. Maybe something resolved itself - hah! Maybe the flickering will return with a vengeance sometime. Right now, flickering is not happening.