If Your Car Has No Heat, This Is What It Could Mean

Driving a car on a cold day with no heat is annoying and a safety hazard. If it's because of air bubbles, here's what you can do.

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Even in warm climates, a working car heater is much appreciated on chilly mornings. And in cold climates, a properly functioning heating system is essential.

A malfunctioning heater blowing cold air prevents the defroster from removing ice and fog from the windshield, creating dangerous driving conditions. Don’t ignore it.

Why Isn’t the Car Heater Blowing Hot Air?

Here are the most common reasons why your car heater isn’t blowing hot air.

  • Low coolant: Whether caused by a leak or water evaporation, low coolant is the most common source of poor heater output.
  • Thermostat: A stuck-open thermostat prevents the engine (and coolant) from heating up.
  • Heater core: A clogged heater core restricts coolant flow, giving up little or no heat.
  • Electric cooling fans: Due to a bad sensor, switch or controller, electric cooling fans running constantly prevent coolant (and the engine) from reaching operating temperature.
  • Heater (hot water) control valve: Some vehicles have a heater control valve that only opens when the temperature control is set to hot. A stuck-closed valve prevents hot coolant from flowing through the heater core.
  • Blend doors: An inoperative temperature control, blend door or a failed (or out of calibration) blend door actuator stuck in the cold position prevents heated air from entering the passenger compartment.

If your vehicle was serviced recently, there could be another reason. If you had the fluids topped off or the radiator flushed with new coolant, or if you have a defective radiator cap, air bubbles could have made their way into your cooling system. Air bubbles keep coolant from circulating through the cooling system, including the heater core.

Here’s how to solve the air bubble problem yourself and get your heater working again. Don’t laugh: Like a baby, your cooling system needs to be burped.

How to Burp a Vehicle’s Cooling System

“Burping” the cooling system removes trapped air. Specifics depend on your vehicle’s make, model and year, and whether it has a coolant reservoir or surge tank, but burping air from your coolant system is usually an easy fix. Be patient, though. The process takes some time to complete.

A coolant reservoir and surge tank do the same thing. As hot coolant expands, excess coolant flows into the reservoir or surge tank. As the engine cools, a vacuum draws the coolant back into the radiator. Surge tanks are placed at the highest point in the cooling system. Coolant is added here rather than the radiator and sealed with the pressure cap.

Safety first

Before removing the radiator cap, let the engine completely cool. Coolant reaches 220 F, even hotter under pressure. When working on the cooling system, always keep fingers, tools and clothing away from the cooling fan (which can run with the engine off) and drive belts.

Tools and materials

  • One gallon or more of coolant (a 50/50 mix of water and anti-freeze).
  • Spill-proof funnel. This keeps coolant from bubbling out of the radiator when air begins to “burp.”
  • Drain pan, placed under your car to keep coolant from spilling.
  • New radiator cap if the existing cap shows signs of wear or rust, or is more than three years old.

Step 1: Add Coolant

With coolant system bleeder valves:

Check if there are bleeder valves (AKA screws), similar to a brake caliper bleeder valve, on the radiator, thermostat housing or intake manifold.

If there are valves, with the engine off and cool, remove the radiator cap and open the valves. Insert the spill-proof funnel in the radiator and add the pre-mixed coolant until escaping air stops hissing and bubbling and a steady stream of coolant flows from the valve. Then go to Step 2.

Without bleeder valves

With the engine off and cool, remove the radiator cap and insert the spill-proof funnel in the radiator. Fill the radiator with pre-mixed coolant until there’s about two inches of coolant in the funnel. Fill the reservoir to the “Full Cold” mark.

Step 2: Start the Engine

Start the engine and set the heater temperature control inside the car to high and the blower motor fan on low. This allows the coolant to reach operating temperature faster by not removing heat from coolant through the heater core.

Let the engine run. Be patient; it can take 15 to 20 minutes for coolant to reach operating temperature. Air bubbling in the funnel means trapped air is being purged.

Step 3: Check the temperature gauge

With the radiator cap off and engine running, keep the funnel filled with coolant and check the instrument panel temperature gauge. Temperature readings should be normal once air has been fully purged from the coolant.

Step 4: Finish Up

Keeping the coolant level full, let the engine run for another 10 to 15 minutes, or until you’re satisfied all the air has been removed. Carefully remove the funnel. The coolant will be really hot. Spill-proof funnels have a plunger that prevents any coolant still in the funnel from spilling when removed from the radiator.

Fill the radiator to the top and reservoir/surge tank-to the “Hot Full” mark. Replace the radiator cap, turn off the engine and let it cool. After the engine completely cools, check the reservoir/surge tank level. It should be at the “Full Cold” mark. Add coolant, if needed.

Alternate step: Jacking up the front end

It may be necessary to raise the front of your vehicle to force air pockets out of the cooling system.

Using a jack and jack stands, safely lift the front of your car until the radiator fill neck is higher than the engine. Secure the jack stands. Once raised and firmly sitting on the jack stands, follow the steps above to thoroughly burp air out of the cooling system.

The Final Word

Address car heater problems right away. If coolant levels keep dropping or air bubbles come back, it’s time to see your mechanic.

No heat on frigid days places you and passengers at risk if the windshield fogs up or ices over. Air trapped in a cooling system can also cause engine overheating, resulting in premature engine failure.

Flushing fluids at the manufacturer’s suggested service intervals is the easiest and least expensive way to extend the life of your vehicle.

Bob Lacivita
Bob Lacivita is an award-winning ASE and General Motors auto technician, educator and freelance writer who has written about DIY car repairs and vehicle maintenance topics. His work has been featured in The Family Handyman, a Reader's Digest book and Classic Bike Rider magazine. He has been a career and technical educator for 25 years teaching automotive technology, as well as writing state, federal and organizational foundation grants. He also helped design a unique curriculum delivery model that integrates rigorous, relevant academic standards seamlessly into career and technical education.