Cpu fan not detected - yet fan is working

Thanks.  I'm sure that it is the fan connection because that is where the old fan was removed, and it is also shown on Dell's diagram.  I've read that "I could remove heat sink and fan, clean cpu and heat sink with solvent, apply Quick Silver or other heat transfer grease (per directions) and reinstall. Be careful not to damage the CPU"  I think I will try it at some point.  Maybe this evening.  But I'm sure open to any other suggestions!!!

Is the replacement fan the same as the one removed. Usually the fans that can detect rotational speed of the fan have a 3 wire connection to the motherboard, If its only 2wire it probably won't detect the speed.

Thanks!  Yes, it is a  fan and shroud model specifically for a Dimension 2300 et al, and it does have 3 wires.

Thanks smiffy13.  I followed your instructions (num lock, cap lock, &  scroll lock on lit green) Alt+E  (Got no beep).  And it didn't correct the fan not detected message.  I couldn't find clr-eisa or clr-nvram on the motherborad.  I did change the jumpers and clear CMOS.  No change.  The fan is spinning; the computer is working just fine.  But still that pesky message.

Well, either the motherboard sensor isn't working or the fan isn't transmitting the rotational data.   If your BIOS has the option (some do, some don't) you could simply disable the fan warning.

I guess I'll admit defeat - and tell my friend, the computer owner, that I can't eliminate the alert; but that his computer is working OK.  I think I'll tell him to be sure to keep his important files backed up on a regular basis.  Thanks for everyone's help.

Does the BIOS have a setting to disable the warning?    Is it only a bootup warning?  (i.e. is it quiet afterwards?)

I've looked and looked for that one.  There is a place to clear the event log, and to verify the log has been read (which also clears the log.)  I've done that.  But a new warning is generated the next time I boot up. Nope, there is no option to disable the warning in the BIOS.  

Yes, it is only a bootup warning - no further alerts or warnings after the computer is booted up.