* Blend door actuator or cable: This is the most common cause. The blend door controls the mix of hot and cold air. If the actuator (the motor that moves the blend door) is broken or the cable is disconnected or frayed, it can't direct the hot air into the cabin, even though the core is hot. This is often accompanied by the inability to switch between hot and cold air.
* Blend door itself: The blend door might be stuck in the closed position, preventing hot air from entering the cabin. This could be due to mechanical failure, debris, or vacuum leaks.
* Vacuum leaks: The blend door is often controlled by vacuum. Leaks in the vacuum lines will prevent proper operation of the blend door, resulting in no heat. Check all vacuum lines for cracks, holes or disconnections.
* Heater control valve (if equipped): Some systems use a valve to regulate coolant flow to the heater core. If this valve is stuck closed, no matter how hot the core gets, no heat will reach the cabin.
* Faulty blower motor: While the core is hot, the blower motor might not be working to push air through the heater core and into the vents. Listen carefully to see if the blower is running. If not, it could be a blown fuse, bad motor resistor, or a faulty blower motor itself.
* Clogged heater core (a rare possibility given your description): While you say the core is extremely hot, it's *theoretically* possible that only a small portion is extremely hot, while the majority remains clogged and ineffective at transferring heat to the air flowing through it. This is less likely given your description, but worth noting.
Troubleshooting Steps:
1. Check the blower motor: Make sure the fan is actually running on all speeds.
2. Check for vacuum leaks: Inspect all vacuum lines related to the heater control system.
3. Manually operate the temperature control: See if you can manually feel the blend door moving as you change the temperature setting. This will help determine if the actuator or the cable is the problem.
4. Inspect the blend door actuator: If accessible, visually inspect the actuator for signs of damage or binding.
5. Check fuses and relays: Refer to your owner's manual for the appropriate fuses and relays related to the heater system and blower motor.
Starting with the blend door actuator and vacuum lines is usually the most efficient way to troubleshoot this issue. If you're not comfortable working on your vehicle's HVAC system, it's best to take it to a qualified mechanic.
