* Blend Door Actuator Problem: This is the most likely culprit. The blend door actuator is a small motor that controls the flow of hot and cold air to the different vents. If it's malfunctioning on the passenger side, it might be stuck in the cold air position or not working at all. This is often a relatively inexpensive fix, but it does require some disassembly to access the actuator.
* Blend Door Issues (Beyond the Actuator): The blend door itself could be broken or jammed, preventing it from properly directing hot air to the passenger side. This requires more significant disassembly and potentially replacement of the blend door.
* Clogged or Restricted Heater Core: While less likely to affect only one side, a partially clogged heater core can reduce overall heat output. If the restriction is significant enough or in a specific area, it could lead to less heat on one side than the other.
* Low Coolant Levels: Insufficient coolant in the system will reduce the overall heating capacity. Check your coolant level and add more if needed (using the correct type for your Nissan). A leak could also be the underlying cause of low coolant.
* Faulty Thermostat: A malfunctioning thermostat could prevent the engine from reaching its optimal operating temperature, leading to less heat from the heater.
* Vacuum Leaks: Some heater control systems rely on vacuum to operate. Leaks in the vacuum lines can disrupt proper operation of the blend doors.
* Wiring Problems: A problem with the wiring to the blend door actuator could prevent it from receiving power or the correct signals.
Troubleshooting Steps:
1. Check Coolant Levels: The simplest first step.
2. Listen for Actuator Movement: When you change the temperature settings, listen carefully near the dashboard. You should hear a faint clicking or whirring sound as the actuators move. If there's no sound on the passenger side, the actuator is likely the problem.
3. Check All Vents: Ensure the problem is isolated to the passenger side. If other vents are also blowing cold air, the problem is likely more widespread (e.g., low coolant, thermostat, or heater core).
4. Consult a Repair Manual: A repair manual specific to your Nissan model and year will provide detailed diagrams and instructions for accessing and diagnosing the blend door actuator and other components.
5. Consider Professional Help: If you're not comfortable working on your car's HVAC system, it's best to take it to a qualified mechanic.
It's difficult to diagnose the exact cause without inspecting your vehicle. Start with the simpler checks (coolant level) and work your way toward the more complex issues. The blend door actuator is a common suspect, though, for this type of problem.
