Here's why:
* Blend Door Actuator: This small motor controls the blend door, a flap inside the HVAC system that mixes hot and cold air. If this actuator malfunctions or is stuck, it can only direct air to one position – in your case, the footwell vents. The defrost mode usually has a separate, independent actuator, which is why it works correctly.
* Vacuum System (Less Likely, but Possible): Older vehicles sometimes use a vacuum system to control the blend doors. A leak in the vacuum lines or a malfunctioning vacuum control unit could prevent proper operation of the blend doors. This is less common than a failing actuator.
* Wiring Issues: A short circuit or broken wire to the blend door actuator could also prevent it from functioning correctly.
Troubleshooting Steps (In Order of Ease):
1. Check the HVAC controls carefully: Make absolutely sure you're not accidentally hitting a button that forces air to the feet. Sometimes a sticking button or faulty switch can cause this.
2. Inspect the fuses: Check your owner's manual for the location and rating of the HVAC system fuses. A blown fuse could interrupt the actuator's power.
3. Listen near the HVAC unit: With the AC on, listen closely near the dashboard. You might hear a clicking or grinding sound, indicating a struggling blend door actuator.
4. Professional Diagnosis: The most reliable method is to have a qualified mechanic diagnose the problem. They can access the HVAC system and test the actuator directly, check vacuum lines (if applicable), and identify any wiring issues. This is often the quickest and most efficient way to resolve the issue.
It's unlikely to be a major component like the compressor itself, since the defrost works, showing the system is getting power and refrigerant. The problem is almost certainly in the air distribution system itself. Replacing the blend door actuator is a fairly common repair for this type of symptom and generally isn't overly expensive.
