1. Heater Radiator Clogged
This is especially common in cars that are a few years old. The internal pipes of the heater core are quite narrow. If the coolant used over a long period is of poor quality or not changed frequently, scale, rust, and impurities can easily build up.
These substances accumulate and can eventually clog the narrow pipes. The symptoms are: the engine temperature gauge looks normal, or even slightly high, but the air blowing from the passenger compartment is not hot, or the airflow is weak, and the temperature is low.
How can you initially determine if it’s clogged?
After the engine has warmed up, carefully touch the two hoses connecting to the heater core. If one feels very hot, while the other is only warm or even cool, then the heater core is likely clogged.
However, repairing this is quite complicated. It requires removing the dashboard, taking out the heater core for cleaning or replacement, and must be done by a professional repair shop.
2. Thermostat Malfunction
The thermostat acts like an automatic switch for the engine cooling system. When starting a cold engine, the thermostat closes, allowing the coolant to circulate only within the engine’s internal loop to help it warm up quickly. It only opens once the set temperature is reached, allowing the coolant to enter the main circulation loop (including flowing through the heater core).
If this “switch” malfunctions and remains permanently open, the coolant will prematurely enter the main circulation loop. As a result, the engine temperature won’t rise and will remain consistently low.
The most obvious symptom is that even after driving for 10-20 minutes or longer, the temperature gauge needle won’t rise, remaining stuck in the low-temperature range, and the heater won’t work effectively. This situation is relatively easy to diagnose; simply observe the temperature gauge. Replacing the thermostat is a routine task for repair shops.
3. Insufficient Coolant
Coolant is the “blood” of the entire system. If the level is too low, the circulation volume is insufficient, resulting in less hot water flowing to the heater core and poor heating performance.
Furthermore, over time, the coolant’s antifreeze, anti-boiling, and especially its anti-corrosion capabilities will significantly decrease. This not only makes it easier for scale buildup to cause the aforementioned problems, but it also reduces its own heat transfer efficiency.
The symptoms are usually poor overall heating performance, sometimes accompanied by abnormal temperature gauge readings. Checking it is relatively straightforward: with the engine cold (after it has cooled down), open the hood and check the coolant level in the expansion tank, ensuring it’s between the minimum (MIN) and maximum (MAX) marks.
Then check the coolant itself. If it becomes very cloudy, black, or contains floating impurities, it needs to be changed. A crucial reminder: never add ordinary tap water or well water to the coolant for convenience or as a temporary fix! Doing so will produce even more scale, a major culprit in clogging the heater core.
