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 bend 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


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 (Updated 2015-01-16)