CPU fan not detected

[FreeFall79]

Hi all. I noticed a few other threads on this but since my case is a bit different, I decided to start a new one. I have a Dell Dimension 2350.
At startup, I get a screen with the alert:CPU fan not detected.
Obviously I checked the back of the desktop and 2 fans are visible. Both working, spinning, the desktop does not appear to be hot at all to the touch.
Also, I did not open up the case at all, changed any hardware, or dropped it. This message just seemed to appear. If I can think of anything different that I have been doing, it is that I have been putting it on standby to reduce energy consumption (yay for electric bills =( ) So, that is the only difference.
I can also press F1 and get right to Windows, operate all programs. It is an older comp but not bogged down with anything really, only 512 RAM and I like to keep some programs for security running, lately it is a little slower at times.
But I was wondering if I need to open up my computer or not since I see the 2 fans from the back working. Why is the alert coming up?
I will appreciate any and all feedback. Thank you.
-Melissa

[Xylon]

CPU fan not detected is problem with cpu fan ,so first check for connections and wire that working proper check in bios for cpu fan option and its setting make save settings for bios and then see if its working or not also try this Fan not detected??

[Hardwareman]

CPU fan is connected to mother board with a three wire connector. May be in your case wires are not connected properly or not aligned properly. The correct setting is first black, second red and third blue. Check the connection setting. I think this will help you.

[Killen]

It may happen that your fan as a hardware itself will be not working conditionally. For this kind of issue you may try some other fan connecting to your computer or connect your fan to some other computer, by which at least you would come to know that there is no problem inside your computer settings or wire connections.

[Fabian]

You said your fan is working properly but the error come up on boot CPU fan not detected. This may be due to your bios is not detected the fan. First you will change your bios setting default or you may try this method if you have some hardware knowledge.You will open your System Unit, Remove the CMOS battery, wait for 4-5 minutes, then put the battery back, and start your system, it will display CMOS setting has been changed or Incorrect then just press F2. This will change your bios setting to default.

[FreeFall79]

Ok today I removed the case and did not see any wires that were not secure. Nothing inside has been replaced or toyed with. the fan that has the green casing inside the dimension 2350 seems to spin and work fine. the fan above that spins and works fine too. I am supposing these are the only 2 fans in the computer. If there are any unseen fans, I do not know. So, with these 2 fans operating, I still get the "CPU fan not detected" message at start up. Is this just something I should ignore and just work on not getting the message or perhaps it is caused by too much dust. I do not have a can of compressed air yet but I intend to clean the fans anyway. Any suggestions on why I have this message while the fans are still spinning or is it the dust. I welcome any feedback. Thank you
-Melissa