The heater in a car relies on the engine to function. The engine produces heat, and the coolant system in the car circulates this hot coolant around the heater core. It is essentially a tiny radiator that blows air over it, warming up the air before it enters the cabin of the vehicle. Your engine is running and producing heat when you’re driving, so your heater will work as designed. However, when you’re idling or parked with the engine off, there’s no heat source for your heater.
Check the first step if your engine is heated up and still not have heated air.
#1 Check the Coolant Level
The cooling system in a car needs to have the proper amount of coolant (also called antifreeze or engine coolant) to function properly. If the level is too low, heat will not distribute properly. The hoses that carry fluid to and from the heater commonly take fluid from the top of the water pump or block. If there isn’t enough coolant, then it won’t be able to circulate properly and will only work when driving because that’s when the water pump moves enough fluid through the hoses.
STEP 1: The first step in filling up your car’s coolant is to find where it is stored. It will be a small reservoir near the engine in most cars.
If the coolant level is low, open the hood and locate the reservoir (usually a clear plastic tank) that holds the coolant. Once you have found it, unscrew the cap and set it aside.
STEP 2: To add coolant to a car, ensure you have the correct type of coolant. You can use diluted coolant by itself or a 50/50 mixture of concentrated coolant and distilled water. There are also many coolant brands available.
Then, slowly pour in your chosen coolant until it reaches the “full” line on the reservoir. Do not overfill it!
STEP 3: When the coolant rises to the “cold fill line,” replace the cap and tighten it until you feel it click. Make sure you screw on the cap tightly so that no coolant will leak.
#2 Check the Horses
As you may know, there is a coolant pressure cap. The pressure cap is a very important component of the cooling system, as it helps maintain the correct level of pressure. If your radiator horses are soft, the pressure within the system is low. That could be why your heater only works when driving – because there’s not enough pressure.
To fix this problem, you’ll need to replace your coolant pressure cap with a new one.
Other than that, it could also be because of block or kink heater hoses. In such cases, the hose needs to be replaced.
#3 Scan the Car
Nowadays, most cars have an electronic control module (ECM) that monitors and controls various engine functions. One of the things that this module does is keep track of the operating temperature of the engine coolant. If something is wrong with the thermostat, it can often be diagnosed by scanning the ECM. If the problem is with the thermostat, you will have to replace it.
The cost of replacing a thermostat in a vehicle can vary depending on the vehicle’s type and model. However, the average unit usually costs between $20 to $100. If you need to take your car to have it done by a professional mechanic, you could be paying upwards of from$100 to $ 500 for their services.
Conclusion
Your heater only works when driving because there isn’t enough coolant in the system. Adding more coolant to the reservoir may solve the problem. However, if you add coolant more frequently, there may be a leak somewhere in the system that needs to be repaired.
Other than that, it could also be because of block or kink heater hoses. In such cases, the hose needs to be replaced. Or, it could be an issue with the thermostat that can often be diagnosed by scanning the ECM. If the problem is with the thermostat, you will have to replace it.
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