* Blend Door Actuator/Cable: This is the most likely culprit. The blend door controls the mixing of hot and cold air. If it's malfunctioning (stuck closed on the cold air side, broken cable, or faulty motor), you'll get hot coolant circulating but only cold air blowing out.
* Heater Core Restriction/Blockage (less likely): While you have hot lines, a severely restricted heater core *could* prevent sufficient heat transfer. However, this is less probable given you're seeing hot lines; a fully blocked core would usually result in lukewarm or cool lines as well.
* Vacuum Leaks (less likely): Some climate control systems rely on vacuum for operation. Leaks in the vacuum lines could affect blend door operation.
* Faulty Blower Motor (less likely): A weak blower motor might not be able to move enough air to feel any heat even if the blend door is open. However, you should still feel *some* warmth with a partially functioning blower.
* Wiring Issues (less likely): While less common, a wiring fault could prevent the blend door actuator from receiving power or signal to operate correctly.
Troubleshooting Steps:
1. Check the blend door actuator: This often requires accessing the HVAC system (sometimes behind the glove box or under the dash), and may require removing panels. Inspect visually for obvious damage or binding. If you're mechanically inclined, you can sometimes manually operate the blend door to see if it moves freely and affects air temperature.
2. Inspect vacuum lines (if applicable): Check for any visible cracks or disconnections in the vacuum lines that control the climate system.
3. Check the blower motor: Ensure the blower motor is running at full speed. A weak motor may still work, but insufficient airflow will lead to poor heat output even with a functioning heater core and blend door.
4. Consider professional diagnosis: If you're not comfortable working on your vehicle's HVAC system, a qualified mechanic can diagnose the problem accurately and efficiently. They have specialized tools and experience to identify the issue without causing further damage.
It's crucial to address this issue quickly, especially in colder climates, to ensure your comfort and safety. Remember to always consult your vehicle's repair manual or seek professional help if you're unsure about any repair procedure.
