Both of my Ford’s, now after I replaced the resistor (Thank you Mr. Meehan) in the 2001 Ford Focus, the A/C worked on all settings for a couple of days, now the fan setting 4 does not work at all, the A/C shuts off and no fan turns, only works in 1 thru 3 settings. But I am not done yet, my 1994 Ford Escort started this a year ago, I replaced the A/C controls and it still does not work on fan setting #4, nothing comes out, nada, just like the Focus now, fan setting #4 nada, but A/C works on all other settings (#1 thru #3)
Here’s what you run into. Often a resistor or fan switch failure has a root cause, and that cause is often a worn and dragging blower motor.
Worn and dragging means the current draw is much higher and this generates much more heat, which in turn burns up the resistor or fan switch.
The No. 4 setting, or the high speed, does not run through the resistor and in some cases has a separate fuse for the high speed operation.
Understand I’m theorizing here without a schematic in front of me.
In the event the blower circuit uses only one fuse then I would suspect a failing fan switch in both cases, with a dragging blower being responsible for the failures.
An inductive ammeter can be used to check blower motor current draw. Just ballparking but I think the high speed current draw should probably be around 4, maybe 5 amps. If it’s in the 7-10 range then the blower motor is on the way out but not before taking other things with it.
Also, wire connectors at the resistor AND the fan speed switch should be closely inspected. These often burn to some degree and this means a bad connection. A bad connection in turn means a high resistance, which then means more current, more heat, etc.