I have a FSM for a 99 sitting right here. According to this, straight from the manual, "the P1491 is for an open or shorted condition detected in the radiator fan control relay control circuit. This includes PWM solid state relays."
Did you check wires from the computer to the PDC? Temp sender wires?
Can you get a pick down to the pins for the fan relay and bend them a little bit to see if the better contact with the relay will fix the issue?
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