How to Fix a Heater

Step by step guide on how to fix your heater, this information pertains to most cars, trucks and SUV's.

Difficulty Scale: 4 of 10

Tools and Supplies Needed

  • Flashlight
  • Screwdriver set
  • Protective gloves and eye wear

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 engine thermostat control the rate in which the engine heats up. If this unit is stuck open the engine will stay at a low temperature which created little to no heat for the heater. Visit - Thermostat Replacement

Engine Thermostat

Step 3 - 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. Please Visit - Blend Door Actuator Replacement

Blend Door Actuator

Step 4 - this problem happens more than I thought at first, but after reviewing many posts with the same problem the fix was relatively simple. With the engine running at full temp jack the front of the car up and let it run for 2 minutes then go to the rear of the car and do the same thing, this will burp or purge the air bubbles locked in the heater core and allow the heater to work again.

Step 5 - 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 6 - 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 7 - 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 8 - 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 9 - If no air is being produced from the vents while the heater is in operation inspect the blower motor system.

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