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Is Your Car Heater Toasty? If Not, Ask These Six Questions

Air Conditioning & Heat, DIY, Guides  /   /  By Gary Lieber

Winter is approaching fast—so now is the time to make sure your car’s heater is up to the challenge.

When you crank up the temperature control and blast the air, it can take a few minutes before the cabin warms up. That’s because heat from the engine first needs to build up—and then coolant must move through the engine, get warmed up, and circulate to the heater core where it can be blown into the passenger compartment.

Unfortunately, sometimes no matter how long your car is running, you still can’t get that nice toasty feeling. If so, then ask these six questions.

Make sure that your car’s coolant is at the right level.

1. Is there enough antifreeze and water in the system?

Check the coolant level in your vehicle. If your ride is low on coolant, it will not fill the heater core sufficiently to blow warm air. Check the coolant tank in your engine bay, making sure the coolant is above the minimum mark but doesn’t exceed the designated maximum safe level.
> Read more about coolant issues in this post: Before Winter Strikes, Make Sure Your Car Heater Is Ready

A malfunctioning engine thermostat can cause a heater to run cold.

A malfunctioning engine thermostat can cause a heater to run cold.

2. Is the thermostat working correctly?

If you’ve confirmed that there’s enough coolant, the culprit could be an engine thermostat that’s stuck is in the open position. A malfunctioning thermostat could prevent the coolant from sufficiently heating up. Check for a stuck thermostat by starting your car and then watching your engine’s temperature gauge. If after about 10 minutes, the engine is still reading “cold,” then there’s a decent chance that your engine needs a new thermostat.

3. Is there a coolant leak?

Look under the vehicle for leaks on the ground. If you find any—and if your heater has either a sweet or odd smell—then the heater core could have a leak and need to be replaced.

Use the bleeder valve to let out air trapped in the cooling system.

Use the bleeder valve to let out air trapped in the cooling system.

4. Are there air pockets in your cooling system?

In the process of replacing the coolant in a vehicle, air can get trapped in the system—preventing the coolant from properly circulating. Here’s how to check. Let the engine warm up long enough for the cooling fan to come on. Locate the bleed screw on the radiator (or on the thermostat housing on the engine). Open the bleed screw a quarter-turn. If you hear a hissing sound, there is air trapped in the system. Leave the bleed screw opened a quarter-turn until coolant starts coming out—and then close the bleed screw.

A clean cabin filter will make sure your car stays warm.

A clean cabin filter will make sure your car stays warm.

5. Is your HVAC cabin filter clogged?

Cabin filters should be replaced every 15,000 miles. They can clog up with dirt and thereby prevent adequate airflow through the heater core. Make sure that your cabin filter is clean with sufficient airflow.

6. Are the heater controls and actuators working correctly?

Depending on the vehicle, the controls for the HVAC (Heater Ventilation Air Conditioning) system may consist of metal cables, vacuum hoses, switches, or electrical actuators. With the temperature set to the hottest level, check to see if hot air is blowing. If not, then the heater core valve might not be opening. If the valve uses a metal cable or vacuum hose, make sure the cable and hose are not loose or broken. Move the valve by hand, touching the coolant hoses that enter the firewall to make sure that they are both hot. If one is cold, you should consider replacing the heater valve. On older cars, this could also mean that the heater core is blocked by debris and needs replacement.

About the Author

Gary Lieber is a regular contributor to the eBay Motors Blog, Clean Fleet Reports,, and He likes to modify his cars by adding features that are unique to the marque. His Porsche 911 is a multiple Concours winner including Top 911 at Porsche Parade in 2005 and 2015. His Volkswagen GTI has been tricked out with engine management upgrades and style enhancements not commonly available on this side of the pond.

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