Heater Not Working

Step 6 - The heater core is responsible for converting hot coolant into heated air, if this core becomes plugged it will not operate correctly. To check for a plugged heater core, start the vehicle and allow the engine to warm up to normal operating temperature while turning the heater to its hottest setting. Grasp both heater hoses, they both should be hot, if only one is hot, the core could be plugged or the heater control valve is malfunctioning.

Heater Core

Step 7 - If no air is being produced from the vents while the heater is in operation inspect the blower motor system.

Blower Motor

Step 8 - An air cabin filter helps clean the air before it enters the passenger compartment, when this filter becomes plugged it stops the airflow and needs to be replaced.

Replacing Air Cabin Filter

Helpful Information

Sometimes engine coolant level can be low without the engine overheating, especially in winter conditions. A heater control valve can be actuated by either a vacuum line or control cable. To warm the passenger cabin, the heater system makes use of heat generated from the engine by blowing across the heater core and onward through the system ductwork. When a heater core develops a leak, there are usually two indications, steam could be produced from the vent system and is accompanied by a pungent odor, or the passenger floor can become wet from leaking coolant, these are signs the core needs replacement.

Related Car Repair Information


Written by
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35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2015-04-23)