If you start your car on a cold morning and only cold air comes out of the heater, check the following steps carefully before you panic. In most cases, the cause can be narrowed down with a simple inspection.
1. First, check the coolant (antifreeze) level.
The heater uses the heat from the coolant heated by the engine to generate warm air. If the coolant level is low, heat will not be transferred properly. However, never open the coolant cap when the engine is hot. Check the level in the auxiliary tank only after it has completely cooled down, and if it is low, add coolant of the correct specifications for your vehicle. If the level continues to drop, there may be a leak, so please have it serviced.
2. Try changing the fan (blower) wind speed.
If almost no air comes out, it only operates at specific speeds, or makes strange noises, the problem may be with the blower motor, resistors, or fuses. If air is coming out but it is not warm, the proper order of things is to suspect the heat transfer components (coolant, thermostat, heater core) rather than the blower.
3. Check if the cabin filter is clogged.
A clogged cabin filter reduces airflow and lowers heating efficiency. If there is excessive dust or an odor, check the replacement cycle (confirm the recommended interval for your vehicle model) and replace the filter. The filter is usually located behind the passenger-side glove box.
4. Please also test defogging.
When switching to windshield defroster mode, check to ensure that air is properly directed toward the glass and that no objects are blocking the vents or ducts. Since the air conditioner often operates to dry out moisture during defrostering, it is helpful to turn it on if the air conditioner button is currently off.
5. If it's still cold, take it to a repair shop.
If the vehicle does not warm up even after checking the coolant, blower, and filter, or if the engine temperature gauge fluctuates abnormally, it could be due to the thermostat not opening properly or an issue with the coolant circulation or heater core. At this stage, do not attempt to disassemble it yourself; instead, have it inspected before winter arrives. A heater failure goes beyond mere inconvenience and can lead to safety issues, such as driving with frost on the heater.
Source: Certified Auto Repair Winter Heating System Inspection Guide — Certified Auto Repair, Minit-Tune Heating & Cooling System Guide — Minit-Tune
This article provides general information, and the structure and maintenance methods may vary by vehicle model. Do not open the coolant system when the engine is hot. | 
