Question Dell Optiplex 990 won't boot due to temperature sensor missing


Nov 3, 2015
Hello everyone. I am currently fixing a pc for a friend. It's an Optiplex 990.
Whenever i try to boot it up it hits me with an "Alert! Temperature Sensor missing" message. As well as a "hard drive fan missing" message.. this (unlike other errors i've seen in different pcs) completely denies access to the pc. I can only access the BIOS and that's it. I was thinking that it's something about the motherboard so i would replace it. What do you think?
My friend doesn't want a new PC.. just to fix this one up. Do you think there is anything else needed? Purpose of the pc is simple productivity tasks and perhaps a bit of casual gaming (there is also a GTX card installed). All I need is to be able to boot properly and i feel it's the motherboard's fault

CPU: Intel Core i7-2600 @ 3.4 GHz
4x 1333 Mhz DDR3 Ram DIMMs
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Without a proper diagnosis, I would not blame the mobo. Those warnings may actually indicate failure in the respective parts. Considering that you are seeing two failure alerts, there is a possibility that the PSU is not performing as it should, thus not providing the right voltage to the motherboard, and then subsequently the parts connected to the board. Unless you have a spare 990 laying around from which you can "borrow" the PSU, you'll have to test the PSU voltage.

While I'm not sure why a hard drive needs a dedicated fan, I surmise this is the front panel fan responsible for air intake. So, check all fans for proper operation. Since this PC is not used intensively, you might be able to get away with using a "molex Y splitter" cable for the fan.

As for the temp sensor... you'll have to determine if this failure is the sensor, voltage, or wiring.

A distant third thing to check would be for some stray artifact touching the motherboard, causing high resistance.