It's minus 5 outside, and the heater is barely blowing lukewarm air. A familiar scenario for many drivers, especially with cars older than 7-8 years. Let's look at the 5 causes we see most often at our shop.
1. Clogged Heater Core
This is the number one cause. The heater core is a small heat exchanger behind the dashboard through which hot coolant flows. Over the years, its interior gets clogged with sediment, rust, and old antifreeze. The symptoms are distinctive: the heater warms poorly, or one side blows warm while the other blows cold. At HOT22, we flush the heater core, which resolves the issue in 70% of cases without the need for an expensive replacement.
2. Thermostat Problem
The thermostat regulates the flow of coolant. When it gets stuck in the open position, the engine doesn't reach its normal operating temperature of 85-95°C and stays at 60-70°C. The result: the heater works, but not well enough. A telltale sign is the temperature gauge not reaching the middle of the scale even after 15-20 minutes of driving. Replacing a thermostat is quick and relatively cheap — usually between 25 and 60 EUR.
3. Low Coolant Level
If the antifreeze level has dropped, there isn't enough fluid to circulate through the heater core. The causes could be a leaking hose, radiator, or water pump. Check the expansion tank — the level should be between the MIN and MAX markings. A low level with no visible leak could indicate a head gasket issue — don't delay diagnostics in that case.
Having a similar issue? Contact us for quick diagnostics.
4. Stuck Blend Door
The blend door is the flap that controls the mix of hot and cold air. When it gets stuck or its actuator fails, the heater may only blow cold air regardless of the temperature setting. You'll usually hear a faint clicking sound when turning the temperature knob. The repair requires access behind the dashboard, which takes more time, but it's a solvable problem.
5. Air Lock in the Cooling System
After a coolant change or cooling system repair, air can get trapped inside. The air bubble blocks circulation through the heater core. The symptom is intermittent heating — warm for 30 seconds, then cold for a minute, then warm again. Bleeding the system is a simple procedure, but it requires proper technique.
What Should You Do?
Don't drive all winter with a non-functioning heater — beyond discomfort, it's a safety issue. Without a working heater, windows fog up and visibility drops sharply. At HOT22, we perform complete heating system diagnostics, flush heater cores, and fix problems quickly. Contact us at 089 338 3443.
