According to an article in the Croatian magazine PC Ekspert, mentioned in the enquirer.net, a simple screensaver can toast a GF FX 5800 Ultra board, causing it to reach temperatures of 95-100 degrees Celsius. nVidia have been made aware of this issue and are working on some solution.
The problem, apparently arises when running full versions of 3D screensavers, which the card does not recognize as 3D utilities therefore causing the cooling fan to stop functioning. According to the staff at PC Ekspert, any 3D screensaver will do even Windows' own 3D Pipes but a more powerful one is needed if you are in a hurry to get your board hot. As soon as the screensaver launches the fan starts working but 5 to 10 seconds later it stops. Within a few minutes the card will reach 95-100 Celsius, also causing some distortion on the screen.
Croteam, Croatian games developers of Serious Sam fame, also managed to replicate these findings even when using an nVidia reference board they received from nVidia Developer Relations.
Since most screensavers are setup to run when the users are not in front of their monitors, this problem becomes even more serious. No one dared try this long enough to completely destroy the card but such a possibility is all the more likely since the fan will not kick in again unless the screensaver is exited and another 3D application is started, a fact which raises concern about the overall system temperature as well.
nVidia were informed by PC Ekspert and have confirmed that with some screensavers the fan will stop working. nVidia have however, stated that their cards will not be damaged by such temperatures and that only journalists touch boards with their bare hands, so regular users are not in any danger.
Until some sort of fix for this issue is made available, it would be extremely wise if all GF FX 5800 Ultra owners disable their screensavers. It is certainly not recommended that anyone should try this with their own card since they would be not only endangering their graphics board but compromising their entire system.
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