Hi There, I have a 1999 Suburban that has a fan that works on all settings BUT high. I tore the dash aprt and replaced the switch. It still doesn't work on High. I know this doesn't directly answer your concern but I wriote this in hopes that it eliminates a variable for you
You need to replace blower motor resistor.
if you mean highest setting being the fan speed, then the problem is most likely the fan speed resistor located in the blower motor box area
May need new motor low speed winding may be burned out
if it works intermittently, check the wiring/plug connections under the seat. If it never works, change the relay/ and or fuse. also check the plug behind the seat heater button on the door.
Sounds very much like the speed switch may be faulty, or the ocntacts need cleaning.
You may have a faulty switch that controls the speeds on the a/c blower fan.
sounds like the resistor board has blown, on the highest setting it has no resistors, hence the reason it still works
If you are referring to the blower motor speed, and only the highest setting works, then the problem is a bad Blower Motor Resistor Pack.
It goes to your setting you have saved
A relay is bruned through. I had the same problem. Cost to replace including installation about $200.
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it is set in California in soledad