have a 2000 Nissan Altima that only runs on the fourth speed of the heater/ac blower.
Any ideas ?
Your resistor block is shot. That “drops” voltage at each step of the speed switch to reduce the speed of the fan EXCEPT the highest speed, where the resistors are completely bypassed and the fan motor sees the full 12VDC.
Yep. I agree. You need a new blower resistor. The resistor should be mounted in the HVAC ductwork near the blower.
Something that should also be considered is the blower motor itself. After a lot of miles and wear the motor will drag to some degree.
This means it consumes more current which means more heat.
Another cause of a resistor failure may be something getting inside the heater box through a vent or defroster duct. A piece of trash gets wedged into a spring coil in the resistor, it creates a hot spot on the wire, and burns it apart.