Possible Causes

WARNING: This page is about a different car, the 2003 Mazda Tribute. However, it is still accessible from the selected car via links, so may be relevant.

Possible causes are:

Damaged low/medium/high fan control relay.
Damaged PCM.
Open or shorted LFC/MFC/HFC circuit.
Open VPWR or IGN START/RUN circuit to low/medium/high speed fan control relay.