Car Heaters Demystified: How They Work And How To Troubleshoot Them

The heating system in your car is actually pretty simple in principal. Heated liquid is circulated through a small metal radiator (called a Heater Core) inside the passenger compartment, and a carefully situated electric blower blows air through the heater core as well. As air from the blower passes through the heater core, it becomes heated and is then dispersed throughout the passenger compartment via air ducts, vents, etc.

(See Illustration). If you find that the heater in your vehicle is blowing cool air when it should be blowing warm air, don’t panic. There are some very basic and often overlooked causes for this problem. First, make sure that the engine  coolant (antifreeze)  level in your vehicle is up to where it should be. The same liquid which is used to cool the engine is also used to circulate through the heater core to heat the passenger compartment. If the coolant level is okay, then the next thing to check for is a blockage or restriction in the system which may prevent liquid circulation through the heater core. First, you will need to look under the hood and locate the hoses which connect to the heater core. The best way to do this is to obtain online service information for your particular vehicle which shows the location of all of the heater components and how to service or replace them.

The other method is to simply locate the hoses yourself. You may need to use a flashlight to help you to see. The heater hoses are usually black or red in color, and about the same size as a garden hose. They will be connected to the heater core at the firewall. After you have located these hoses, proceed as follows:

Warning: Excersize due caution during the following procedures in order to prevent yourself from being burned or otherwise injured!

Warm the engine up to normal operating temperature, and then check the temperature of both heater hoses near the firewall. Both hoses should be hot to the touch. If not, then it is likely that a blockage exists within the heater core. If you find this to be the case, then it will be necessary to replace the heater core. Replacing the heater core is generally a difficult task and you may need the help of a professional. But if you do elect to do the job yourself, then be sure to get the necessary service information for your vehicle before you begin. There are also other possible causes of restricted liquid flow through the heater core, such as a defective heater control valve, or a defective engine cooling thermostat. The service information for your vehicle will show the locations of these items and explain how to test and replace them. Good service information is a must.

If you’ve determined that there is sufficient liquid flowing through the heater core and that it is heated as it should be, then the next item to check for is air flow accross the heater core. Set the heater to its maximum temperature and highest blower settings. If you cannot hear the blower running, then either the blower itself is defective or it is not getting power. In this case, some electrical troubleshooting will be in order. Again, you will need service information for your vehicle.

Likewise, if the blower is running, then the problem is likely that the warm air is being misdirected due to a Blend Gate which isn’t functioning correctly. These blend gates are little doors inside the heating system which open and close as needed in order to direct air flow. They can sometimes malfunction and refuse to open and/or close when needed.

If you determine that this is your problem, then you’ll need to follow the troubleshooting procedures for that system which are outlined in your service documents.

Thank you for reading, and I hope that this information helps to get you on the right track to cure a heater problem.