Car heater not working? Reasons why and how to fix it

Your car’s heating system relies on the cooling system to function. It may seem counterintuitive, but the HVAC system uses many of the same parts in contrasting functions. 

While the engine generates heat, coolant circulates through the engine and absorbs the heat, stabilizing your engine’s temperature. While most of the heated coolant is sent to the radiator to cool and be reintegrated into the engine system, some is diverted to the heater core. The heater core acts as a small radiator, and transfers heat to your cabin as required. 

A car’s heating system uses many elements to stay operating. 

Coolant: On top of stabilizing engine temperature, the same coolant used for your engine supports your car heater system. 

Heater hoses: These hoses connect the heater core to the engine’s cooling system. 

Heater core: Prior to heated airflow the heater core heats the coolant as required. 

Blower fans: Your car’s heating system has multiple blower fans that force air from the heater core to your car’s cabin. 

Blend door: The blend door simplifies the integrated elements of your HVAC system by controlling the mix of both hot and cold air to provide you with your desired temperature. 

Thermostat: Monitors heat to ensure desired temperatures are met. 

HVAC control system: Allows you to control and customize temperature and the direction of airflow. 

Coolant shut-off valve: While not all vehicles contain this part, the coolant shut-off valve cuts the flow of coolant from the engine to the heater when the heating system is not in use. 

Auxiliary coolant pump: This part is more commonly used in vehicles where the heating system is in the rear compartment. It assists with ensuring consistent hot air in the cabin. 

Evaporator: An evaporator is typically part of a vehicle’s air-conditioning system. However, it may be integrated into the heating system. It dehumidifies the air before it flows into the cabin. 

Because the car heater system contains many elements, there could be several reasons why your car heater may malfunction. 

Lack of heat often happens when the coolant level is low or contaminated. Hot fluid cannot make its way to the heater core when this happens. In this case, having your coolant checked and replaced is an easy fix. 

A radiator leak could prevent the travel of coolant to the heater core. This could also damage your engine. You can fix this issue in the short-term using a radiator sealant.  

When checking to see if your coolant is the cause, be sure you’ve given it a chance to travel through the car heating system. You should notice warm air after you’ve driven your vehicle for at least five minutes. If you’re still only feeling cold air, though, keep reading to learn what else could be causing the issue. 

Like all hoses that help a vehicle to run, heater hoses are susceptible to damage or leaks. This damage compromises the flow of coolant to and from the engine. With a basic repair kit, you can reestablish the coolant flow and repair the issue. 

If your heater core is broken, you may still feel some heat coming out of your vents. However, if the warm air flow is weak, it’s likely just residual heat from the engine. This issue will require a large repair to prevent significant long-term damage to the engine. 

The heater core can also become clogged when particles build up. In simple cases, you can perform a back-flush by running water through the heater core in the direction opposite normal coolant flow. In more severe cases involving rust or debris, though, a large repair will be required. 

Weaker or cooler air flow may also be caused by your car’s blower fan. If it shorts or breaks, coolant may reach the heater core without blowing into the cabin. Often, the blower motor’s resistor is the issue. 

If you experience issues setting fan speed, this is a strong indication of a faulty resistor. The resistor module is easy and inexpensive to replace. Typically the device will be found under the dash or hood, but refer to your owner’s manual to proceed installing a new resistor module. 

The blend door can become stuck in the cold position, so it won’t be able to mix hot and cold air to produce your selected temperature. The blend door actuator should be completely closed. 

Your vehicle needs a functioning thermostat to regulate the cabin’s temperature. For example, if you set your cabin temperature to 20 ˚C, but hot air keeps blowing, your thermostat is likely to blame. 

You can DIY this problem. However, proceed carefully. First, refer to the owner’s manual to determine the correct thermostat for your vehicle. Then locate the thermostat housing and drain coolant, and carefully replace the thermostat. 

An issue with the car’s electrical system can cause problems in several areas in the vehicle’s operations. This can include the HVAC system. 

Common problems include broken fuses or wiring and malfunctioning heat and AC controls. Use a multimeter to determine if the fuse has blown. Then, following the fuse diagram on the fuse panel cover, remove the correct fuse and replace it with a new one. 

Questions about your car’s heater system? Visit your local NAPA Auto Parts store to speak with an expert.