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What to check first if your car heater is blowing cold

On winter mornings, many drivers are disappointed to find their cars and 4x4s have heaters that are blowing out cold air. However, it isn’t always the heater that’s to blame. The heater’s warmth is derived from the coolant circulating around the engine to cool it down.

In low temperatures, the coolant can freeze unless anti-freeze has been added. In the UK, where temperatures can unexpectedly plummet, most car owners should prepare ahead by ensuring the water tank is topped up and anti-freeze included. Even then, the engine needs to run a few minutes to heat the coolant sufficiently to release enough warmth as it passes the heater. Drivers of vans often speed up the process by placing a piece of cardboard between the radiator and the front bumper to reduce the amount of cold air flowing in. The radiator should never be totally covered and a smaller piece of card used if the coolant becomes too hot.

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Water Levels

When the level in the water tank falls too low, there will not be enough coolant to reach the heater which is usually situated higher than the engine. Restoring the water to the correct level, and including some anti-freeze, should correct the heater’s lack of warmth. If the water level has droppedunexpectedly, it might be a leak in the system which is causing the heater to blow cold. As they age, cars and vans frequently develop small splits in the rubber hoses. These can be temporarily repaired with tape until they can be replaced. Locating the source of the leak can sometimes be difficult, but in cold weather, once the engine has heated up, signs of steam should be seen coming from the leak. Observe the hoses, water pump, radiator and the head gasket. A wet carpet or condensation on the windows can point to the leak spilling into the car interior. If the problem continues unchecked it can lead to overheating and complete engine failure.


With water levels and anti-freeze working correctly, but still only cold air blowing from the heater, the next part to check is the thermostat. This is responsible for releasing the warm coolant around the engine and the heater. It remains switched off until the engine has reached a certain temperature, so it needs to run for a while before being checked. If the temperature gauge dial on the dashboard indicates cold when the engine is in fact hot, then the thermostat is faulty. Fortunately, this is an inexpensive spare part that is easy to fit. If the heater still blows cold when there doesn’t appear to be a problem with the water level or the thermostat, it could be due to a blockage of air in the cooling system. With the engine running, use a screwdriver to turn the bleed port anticlockwise to expel excess air. Should none of these measures help, it could be the heater itself that is faulty, but a new one can be installed relatively cheaply.