* Blend Door Actuator: This is a small motor that controls the flow of air between the heater core and outside air. A malfunctioning blend door actuator can prevent hot air from entering the cabin, even if the heater core is working. Check to see if the controls are working correctly (fan speed, temperature setting). If you can change between hot and cold air using the controls, then this might not be it, but if there's no change it could be broken or jammed.
* Heater Core Hoses: Double-check that the hoses connecting to the heater core are properly connected and not kinked or collapsed. Feel these hoses when the engine is warm; the hose going *into* the heater core should be hot, and the hose going *out* should be slightly cooler (but still warm). If both are hot, there's no circulation through the heater core. If both are cold, there's a blockage or lack of coolant flow to the heater core itself.
* Coolant Level & Air Pockets: Even though you've refilled the coolant, make sure the system is properly bled of air pockets. Air pockets can prevent proper coolant circulation. Consult your vehicle's repair manual for the correct bleeding procedure (often involves loosening a bleed valve on the engine or heater core).
* Wiring/Controls: Inspect the wiring harness to the heater control panel, blower motor and blend door actuator for any damage or loose connections. A faulty switch or relay could be preventing the heater from activating.
* Blower Motor: While you have heat, a faulty blower motor (the fan itself) prevents air from circulating through the vents, regardless of whether the heater core is hot. Check to see if the fan is actually running when you turn the fan speed up.
* Head Gasket (Worst Case): In extremely rare cases, a blown head gasket can cause a lack of heat in the cabin. This usually involves other symptoms, such as white smoke from the exhaust, overheating, coolant in the oil, or low compression.
Troubleshooting Steps:
1. Check the hoses: Are they hot or cold? This is the quickest way to isolate the problem.
2. Bleed the coolant system: Make sure there are no air pockets obstructing flow.
3. Test the blower motor: Does it run on all speeds?
4. Inspect the wiring: Look for any obvious damage or loose connections.
5. Check the blend door actuator: If the control doesn't seem to influence hot/cold air, this is a likely culprit.
6. Consult a repair manual: Your vehicle's manual should provide detailed diagrams and troubleshooting steps.
If you've checked all of these and still have no heat, it's best to take your vehicle to a qualified mechanic for diagnosis. A more advanced diagnostic tool might be needed to pinpoint the problem.
