Unplug the wire to the radiator fan switch. Short the contacts on the wire end together. With the ignition on, the fan should come on. If not, the usual problem is a foreign object stuck in the fan, the fan blades hitting something or the connector on the wire to the fan being unplugged. The fan itself could be bad, but that is rare.
If the fan comes on, do the test hot with the fan wire plugged back into the radiator switch. If the fan worked with the wire shorted but it does not come on shortly after showing three bars and well before four bars, replace the radiator fan switch.