The Optiplex 990 is a replacement for another 990 that I had. My prior 990 had a problem with randomly shutting down, sometimes even before starting to load the OS. How to resolve the issue that is obviously a board issue, purchase a refurbed 990 and swap the drives, video card, and higher quality power supply. I didn't even turn on the replacement 990 until I had done the above. I booted the replacement 990. As it was going through BIOS loading, the computer reported that the air temp sensor wasn't working, press F1 to continue. Pressing F1, the computer booted up fine. Everything loaded and apps are working fine. No more random power off issues like with the previous 990. Rebooting, the replacement 990, the same temp sensor error appeared during initial BIOS loading. Pressing F1, the computer continues to boot and run fine. I swapped out the thermistor which connects to the motherboard for the temperature reading. I get the same error. This issue should have been caught and corrected, or the board should have been rejected during refurbishing. Since the computer does boot, and the issue only occurs during boot or reboot, I'm going to keep the computer and continue using it. Others should be aware that what they receive may not be as fully tested and functional as they need it to be.