Heater Core
Heater Core
The heater core is a small radiator inside the HVAC housing behind the dashboard. Hot engine coolant flows through it continuously. When you turn the heat on, the blower motor pushes air across the heater core and the hot coolant heats the air. That warm air flows into the cabin through the vents. The heater core is essentially a miniature version of the main radiator — thin aluminum or copper tubes with fins — except instead of dumping heat outside, it dumps heat inside the cabin where you want it.
Heater core failure
A leaking heater core produces a sweet coolant smell inside the cabin — that syrupy antifreeze odor is unmistakable once you know it. The passenger side floor becomes wet with coolant. The windshield fogs with a greasy film that is difficult to clean — that film is coolant vapor condensing on the cold glass. Coolant level drops with no external leak visible under the vehicle because the leak is inside the dash. Heater core replacement is one of the more labor-intensive jobs in the shop because the core is buried behind the dashboard inside the HVAC housing. On many vehicles the entire dashboard must be removed to access the HVAC box. Labor times of 6 to 10 hours are common. The part itself is usually under a hundred dollars. The labor is where the cost lives.
No heat diagnosis
Feel both heater hoses at the firewall with the engine at operating temperature. Both should be hot to the touch. If both are hot but no heat comes from the vents — the blend door that directs air over the heater core is stuck in the cold position. Scan the HVAC module for blend door actuator codes. Listen for a clicking or grinding noise behind the dash when changing temperature settings — that is a failed actuator trying to move. If one hose is hot and the other is warm or cool — the heater core is restricted internally and coolant is not flowing through it. Flushing the core with a garden hose in both directions sometimes clears the restriction. If both hoses are cool — coolant is not reaching the heater core. Check the thermostat first — a stuck-open thermostat that prevents the engine from reaching operating temperature is the number one cause of poor heat output. Also check for air pockets in the cooling system — air trapped in the heater core blocks coolant flow. Many vehicles have a bleed valve on the thermostat housing or a high point in the cooling system specifically for purging air.