CPU Fan Not Detected

This is in reference to a Dell Dimension 2350 desktop PC with XP Home SP3.  Recently I began getting a POST error stating the CPU fan was not detected, Press F1 to continue or F2 to enter setup.  Pressing F1 continues Windows start-up normally, and the PC runs A-OK.  I checked the fan for any indication of a problem, and all appears to be normal.  The CPU heatsink is clean, the fan runs normally, and there is not an overheating issue.  Given these symptoms, is this likely a motherboard failure with the fan control...and/or is there a workaround?  Thanks.