If your car's heater isn't working, or the engine isn't heating up as it should, there could be a few sources. Here, we'll have a look into some possible causes, and tell you what you can do about them.
How a car's cooling system works
The coolant system on a car is genuinely pretty straightforward. It's made up of a network of hoses which carry coolant around the engine. The coolant is forced round the hoses by a water pump, and the engine is cooled by the water passing through channels within the engine block.
This heated water then flows through a radiator which transfers that heat to the atmosphere.
A cold engine is inefficient, so to get it to warm up quickly it is fitted with a thermostat. When cold, this restricts the flow of coolant so it only flows around the engine, and prevents it from entering the radiator.
Once up to temperature the thermostat opens and the coolant flows around the whole system. The radiator and thermostat work in conjunction to keep the water at the optimum temperature which is why once your car warms up, the temperature gauge should remain relatively static.
If it the coolant gets particularly hot, the next line of defence is the radiator fan. This increases the flow of air over the radiator, and so increases its effectiveness.
Why is my car not heating up?
There are a few potential culprits for why your car engine coolant may not be heating up:
Coolant level
First off, check the coolant level. If it's low you need to top it up, then work out where it's leaking from.
The gauge/coolant temperature sensor
Is the gauge actually working? Is it reading correctly? Modern cars have a coolant temperature sensor that informs the temperature gauge, but these sensors can fail. However, when they fail; they often flag up a fault code, which you can decipher using a fault-code reader.
Thermostat
A common cause is a faulty thermostat. If it's permanently stuck open then the car could be 'overcooling'. You can remove the thermostat and test it in a pan of hot water, it should open just before it reaches the boiling point – usually around 95 deg C, then close as the water cools. If it's not moving, then replace it!
Air lock
If you've had/have a coolant leak, or have recently topped up the system, then you may have an air lock, preventing the engine coolant from circulating correctly.
To rectify this, set the heater controls in the car to maximum heat, remove the header tank cap and fill to the correct level. Now start the engine without replacing the cap and idle for a few minutes. The coolant level may drop as the thermostat opens.
Keep topped up to the max level. Squeeze the top radiator hose to help air pump around the system with the engine running (taking care to avoid any moving parts, in particular the radiator fan, which could come on suddenly without warning). It should 'self bleed' and once up to temperature replace the cap and test drive.
Why is my car heater blowing cold air?
If your car's heater system is only blowing cold air even when the engine is warm, there are a few potential causes. The heat that comes through the heaters is generated by the coolant being passed through a heater matrix behind the dashboard.
This is like a mini radiator, and the blower blows air over the matrix, and into the car. If it's not blowing hot air check the following:
Coolant/air lock
Is the coolant at the correct level? Low coolant may prevent it from circulating through the heater matrix? Is there an air lock? Follow the bleeding process outlined above, ensuring the heater controls are set to maximum heat.
Heater control valve
When you set the heaters to hot, air being passed over the heater matrix should be ducted into the cabin. If it isn't it could be that the heater control valve is not working and needs replacing, or on older cars a physical flap which alters the path of the air could be broken, or disconnected from the controls.
Thermostat
If the thermostat is jammed open causing the car to overcool, there may not be sufficient temperature in the coolant to provide sufficient hot air. Test, or replace the thermostat, as mentioned above, if you suspect this to be the cause.
Blockage
There are some very thin tubes in the heater matrix, and if it becomes blocked the coolant may circulate as normal around the engine, but not reach the matrix. The cure is to flush the system with a hose, or by blowing compressed air through the matrix if you can access it.