2004 F-150 XL Manual Heater-A/C controls. Heater fan now onlys works on High speed only. It did it all of a sudden with no warning. Is the problem the resistor or the Fan relay and if it is the resistor where is it located??

need to change the resistor board that controls the fan

replace the heater fuse/breaker and, the whole resistor set (each resistor controls a different speed on the heater fan: low,medium, high speed

I would replace the blower motor resistor first then see if that corrects this. The resistor is located next to the blower motor under the dash.

it is the blower motor resistor. I do not know off of the top of my head if it is part of the climate control unit or external for your particular vehicle. However the symptoms match.

check the blower motor fuse, then you can turn truck on put the speed control to high. if fuse is blown replace fuse, if the speed control only works on high, the blower motor resistor needs replacing. It is on passenger side under glove box in close proximity to the blower motor. The fuse is under the hood on driver side. the cover should say fuses/relay on the top of cover, on most ford vehicles you will need to look in the owners manual for the position of the fuse.

This is very likely a bad blower motor resistor. When this resistor goes bad you have only off or high. I hope this helps.

The control is not the problem. Your blower motor resistor is burnt out on two fan speeds.
This is a high failure rate item that the factory has redesigned at least twice for this concern.

There is a plug up under the glove box that loosens and you need to snug it up.

the switch is on the dash with the other a/c controls. does the fan work at all? high speed OK? but the lower speeds dont work? if any speeds other than high dont work, replace the blower resistor.

Check for a blown heater motor supply fuse,
Check operation of heater motor control relay,
Check operation of fan speed control switch,
Check heater motor speed control resistor pack (usually located in the heater ducting - in the air flow),
Check the heater motor runs if power directly applied to the motor- if not motor may be faulty.