1. Blower Motor Resistor: The blower motor resistor is responsible for controlling the speed of the blower motor, which circulates air through the vents of your vehicle. If the blower motor resistor is faulty, it can cause the blower motor to only work on one speed, which is often the highest setting. This could explain why your air conditioner and heat only work on 4.
2. Blower Motor: If the blower motor itself is faulty, it may not be able to circulate air through the vents of your vehicle, even if the blower motor resistor is working properly. This could also cause your air conditioner and heat to only work on 4.
3. Climate Control Module: The climate control module is the electronic component that controls the operation of the HVAC system in your vehicle. If the climate control module is faulty, it may send incorrect signals to the blower motor and/or blower motor resistor, which could cause your air conditioner and heat to only work on 4.
4. Faulty Wiring: If there is a problem with the wiring that supplies power to the blower motor or blower motor resistor, it can also cause the blower motor to only work on one speed, or not at all.
5. Vacuum Leak: If there is a vacuum leak in the HVAC system, it can also cause the blower motor to only work on one speed, or not at all.
To determine the exact cause of the issue, it's recommended to have the HVAC system of your Nissan Xterra inspected by a qualified automotive technician. They'll be able to diagnose the problem and provide the necessary repairs to restore the proper function of your air conditioner and heat.
