Heater Not Working

Step by step guide on how to fix an automotive heater, this information pertains to most vehicles.

Difficulty Scale: 4 of 10

Tools and Supplies Needed

  • Flashlight
  • Screwdriver set
  • Protective gloves and eyewear
Begin with the vehicle on level ground, engine off (cold) in park with the emergency brake set.

Heater Controls

Step 1 - A heater cannot operate unless the engine is full of coolant/antifreeze. If coolant level is low check the system for leaks.

Refilling Cooling System

Step 2 - The heater control air blend door is controlled by a electronic/vacuum/cable which can malfunction causing a problem. Check the door operation by moving the heat control from hot the cold while listening for moment inside the heater plenum. (Note: This works better with the engine off.)

Blend Door Actuator

Step 3 - Inspect heater system vacuum lines especially to the heater control valve. These vacuum leaks can cause the system to not function. If a hissing noise is present under the dash when the engine is running a vacuum leak exists and must be repaired to restore the heater functionality.

Broken Vacuum Line

Step 4 - The heater control valve prevents the flow of coolant into the heater core during the air-conditioner function, occasionally this valve will stick not allowing hot coolant to flow, inspect the valve for movement while in operation.

Heater Control Valve

Step 5 - Use a vacuum gauge to test various control tubes which are used to move blend door actuators, also use a hand held vacuum pump to test servo diaphragms by connecting to the vacuum port directly. If no vacuum is present, open the hood and inspect all small vacuum lines from the engine to the firewall and under the dash, this test can be used for the heater control valve as well.

Vacuum Gauge

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
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


Please use our question form if you have a specific question about your car as we are not able to give you a full answer on this page.

Article first published