1. Blend Door Issues:
* Malfunctioning Blend Door Actuator: This is a common culprit. The actuator is a small motor that controls the air flow direction (to the passenger side or driver's side). If it's broken or jammed, it might not be directing cool air to the passenger side, even if the system is producing cold air overall.
* Broken or Dislodged Blend Door: The blend door itself could be physically broken or has come loose, preventing proper air distribution.
2. AC System Problems (Affecting both sides, but might manifest unevenly):
* Low Refrigerant: The most common cause of poor AC performance overall. A leak in the system will reduce cooling capacity, and it might affect one side more noticeably than the other due to air distribution patterns.
* Compressor Issues: A failing compressor won't produce sufficient cold refrigerant.
* Condenser Problems: A dirty or damaged condenser (the radiator-like component in front of the radiator) reduces efficiency.
* Evaporator Issues: A clogged or damaged evaporator (inside the dashboard) will limit cooling.
* Expansion Valve Problems: This component regulates refrigerant flow; a malfunction will impact cooling.
3. Electrical Problems:
* Blown Fuse: Check your owner's manual for the relevant fuse(s) controlling the AC system.
* Faulty Wiring: Damaged or loose wiring to the passenger side components could prevent them from functioning.
* Problems with the AC Control Panel: A problem with the switches or the control module itself could prevent the passenger side from activating.
4. Other Possibilities:
* Blocked Air Vents: Check that the passenger-side vents aren't obstructed by debris.
* Vacuum Leaks (for some blend door systems): Some systems use vacuum to control blend doors. Leaks in the vacuum lines can cause improper operation.
Troubleshooting Steps:
1. Check the obvious: Ensure the passenger-side vents are open, and there's no debris blocking them.
2. Check the refrigerant level: This usually requires a professional AC technician with specialized gauges. Low refrigerant points to a leak.
3. Listen for the compressor: When the AC is on, you should hear the compressor cycling on and off. If it's not, there's a problem with the compressor or its electrical supply.
4. Inspect the AC control panel: Look for any signs of damage or malfunction.
5. Check the fuses: This is a quick and easy step to rule out a simple electrical problem.
6. Consult a repair manual: A repair manual specific to your 2002 Mercury Mountaineer can provide detailed diagrams and troubleshooting steps.
Recommendation:
Unless you're comfortable working on automotive AC systems (which involve working with refrigerants – requiring proper handling and disposal), it's best to take your vehicle to a qualified mechanic or AC specialist for diagnosis and repair. Trying to fix AC problems without the right knowledge and tools can be dangerous and expensive.
