I have this Dell Dimension 8400 with XP. Over a year and a half ago, I started to get "Previous Fan Failure" error messages now and then upon boot up...it didn't happen all the time. Then the error messages went away for a long time, including all last summer when my apartment was hotter. Right now it is 72 degrees in here.
However, they have started in again recently and sometimes the message is "CPU Fan Failure". There are two fans on this model, and I guess the larger one toward the bottom is the CPU fan. It appears to be turning at an adequate clip. There is a little dust on the inlet side of the fan blades, but the fins inside look to be pretty clean. I am not sure just how this fan assembly works, but it is covered by a green plastic protector which limits any access to it; however, looking at the interior view of the cooling fins, they are clean. Opened up the case and tried to clean up the insides. I believe the mobo is the bigger one, and it lays vertically and looked quite clean. A board that lays horizontally had some dust on it and I did my best to get as much dust out of there as I could.
Put it all back together and still got the same error message...Previous Fan Failure. The system seems to operate normally once booted up and no more error messages occur even if the computer stays on for a long time. I was looking at a few links on Google and some say to check the BIOS...that is shows the CPU fan RPM. Supposedly there is also an option in there to turn off these cryptic error messages...However, I have no idea how to pull this BIOS page up. Really leery of making any changes there.
I am not sure how to get the green plastic cover off the fan assembly or if it would do any good to do so, since it looks fairly clean. Just wondering if I should just ignore this message as the computer seems to operate OK in spite of it. The computer in question is 4 years old and is my backup machine, but would like to use it as long as it boots up and runs. Thanks for any advice.