[andyeb]

My iMac clearly has a hardware fault, because there is video corruption which follows the cursor around the screen. It doesn't appear in screenshots and persists when the system is booted from both the Snow Leopard install DVD and the Techtool Pro 5 DVD. I've tried all the usual tricks including zapping PRAM etc. It's covered by AppleCare (thankfully) and Apple have arranged a hardware repair - it's being picked up tomorrow, but in the mean time I thought I'd run a few tests...

However, the Techtool Pro video tests come back clean and I don't see anything happening on the 2nd display I have attached to my Mac. If it really was testing the entire video RAM, wouldn't I see activity on both displays?

This makes me wonder if these video tests are just snake oil; is it just drawing colours to the screen and not actually checking the video RAM at all?

Has anyone ever seen a Mac fail the Techtool Pro video test?

Perhaps Micromat need to make the video test more "convincing" on dual screen setups?

[reader50]

The TechTool tests don't seem to catch many faults. Also, some of the video patterns drawn on screen are for your benefit. So you can see if a color doesn't appear, or if you have dead/hot pixels.

Have you tried the Apple Hardware Test? Your specific problem might not be detected either way - it can be caused by a loose GPU heatsink or video RAM not quite keeping up with the VRAM clock.