Car Running Hot But Heater Blowing Cold

Written By The Motor Guy

Certified ASE Master Automobile Technician

Last updated February 14, 2023

Did you know that if your car is running hot but the heater in your car is only blowing cold air, then there may actually be a problem with the engine cooling system?

In most cars, the heat that you get inside the car actually comes from the engine. Heat is transferred to the cabin of your car via engine coolant flowing through the heater core.

If the car is running hot but the heater is blowing cold then there is usually a problem with one or more parts of the engine cooling system. The fault could be caused by a bad heater core, a faulty blower motor, low engine coolant levels, or even a faulty coolant thermostat.

Why is my car running hot but the heater blowing cold air (6 Reasons)?

It’s important to get to the bottom of problems with the cooling system before they get too bad. If the engine is allowed to overheat, then this could cause expensive damage to the engine.

Here a few common reasons why your car heater is blowing cold air even after the engine has heated up.

Reason 1: faulty heater blower motor

A faulty heater blower motor is a common reason for the heater in your car blowing cold air or not blowing air at all. When the blower motor is working properly it sucks air into the climate control system via the cabin filter. If you’ve got the heating turned up in your car then this air then passes through the heater core where it is heated by the hot engine coolant.

If the blower motor is not working or is only turning slowly then the amount of air entering the heater core and coming out through the air vents will be significantly reduced.

It’s not uncommon for heater blower motors to burn out, especially if you don’t change the cabin filter regularly. The can also fail if the heater motor relay fails or if there is a problem with the wiring or fuse feeding the motor.

Reason 2: Bad heater core

The heater core is responsible for heating the air that is sucked into the climate control system by the blower motor. It is part of the engine coolant system and basically works like a small radiator behind the dashboard. If it is not working properly then the air coming from the vents will be cold.

The coolant that is used to cool the engine can also flow through the heater core, which is usually located behind the dashboard. The heater core is like a small radiator. As the hot coolant passes through the heater core, the heat is transferred to the air that is about to be circulated into the cabin with the aid of the heater fan. 

The heater core can fail in a number of ways. One of the main reasons for heater core failure is a build-up of dirt in the coolant system that blocks the heater core feeder pipes. This is primarily caused by not changing the engine coolant when necessary.

Old coolant will lose its rust-inhibiting abilities causing dirt and metal particles to build up in the coolant as it is pumped around the engine. These contaminants can ruin the heater core causing it to become blocked.

Another reason a heater core can fail is due to lack of use. In most cars, coolant will only flow through the heater core when the heating is switched on. If you live in a warmer climate, or just don’t use the heating that much in your car, then the heater core is less likely to be used and could have coolant sitting in it for long periods of time. This can cause gunge and dirt to build up in it and its pipes, causing them to rust and leak over time.

Reason 3: faulty Engine Coolant thermostat

A faulty thermostat may block the flow of hot coolant to the heater core, stopping it from heating the air.

Most engines are fitted with at least two engine coolant thermostats. The thermostats on your engine act like taps that open and close, allowing coolant to flow through the radiator and to the heater core.

When the engine is cold, the thermostat should be closed. Cold coolant is kept circulating in the engine until the engine reaches a specified temperature at which point the thermostat opens, allowing coolant to flow through the engine radiator so that it can be cooled.

When you turn on the heat in your car, the coolant is also allowed to flow through the heater core and back to the engine. This is also controlled by the thermostat, and if the engine is not warmed up then the coolant does not flow through the heater core.

A faulty thermostat may block coolant from flowing through the heater core, no matter how hot the engine gets. This could explain how a car is running hot, but the heater is only blowing cold air. In this case, the blower motor is sucking in cold air from outside and passing it through the heater core. However, the heater core is not hot as it is not receiving any hot coolant from the engine.

Reason 4 – low engine coolant

Coolant is needed to absorb heat from the engine and bring it to the radiator at the front of the car and to the heater core in the car. Low levels of engine coolant will mean that the heat from the engine won’t be transferred as effectively to the inside of the car.

If there is a coolant leak, or the coolant has not been topped up in a while, then there may not be enough coolant in the system for it to work effectively. This will also have an effect on the engine temperature. Low coolant is a primary cause of engine overheating, and if the engine is running too hot then the coolant will be redirected to the radiator and away from the heater core. It will also trigger the cooling fan at the front of the radiator, which will cause a further cooling down of the coolant that is traveling to the heater core.

Reason 5 – Faulty cooling fan relay

The engine cooling fan is controlled by the ECU. When the engine temperature is too high, or if the flow of cold air to the radiator is not sufficient, then the ECU will activate the radiator fan via the cooling fan relay.

The cooling fan relay is like a switch. It is wired to the ECU and powered by a low voltage signal. When the ECU sends a signal to the relay it turns on the fan by switching on 12V power from the battery.

If the relay is faulty it can sometimes start to switch on and off the fan when it is not necessary. This can adversely affect the engine temperature causing it to run hot then cold. This has a knock on effect that causes coolant to loose too much heat, causing the thermostats to close when they would normally be open. If the thermostats are closed then coolant cannot get to the heater core and cannot provide warm air to the inside of the car.

The cooling fan relay is a common fault on modern cars and is easily repaired. They are usually located on the cooling fan itself, and can be replaced without replacing the entire fan.

Reason 6 – collapsed hose or coolant blockage

Another very common reason why coolant cannot freely travel to the heater core is because of a collapsed or blocked coolant hose.

As explained earlier, the heater core uses the hot coolant from the engine to heat the air inside of the car. The heater core is usually connected to the engine coolant system via two hoses: a feeder hose that takes coolant from the engine and a return hose that takes coolant back to the engine. If the feeder hose is damaged or partially blocked then this will affect the supply of hot coolant to the heater core.

Blocked or collapsed hoses elsewhere in the coolant system can also reduce the amount of coolant and the temperature of the coolant that reaches the heater core. Even if there is an adequate flow of coolant, if it is not hot enough then it can cause the heater to blow cold air. Coolant temperature can be lowered by blockages in the radiator and feeder hoses that can cause the coolant to pool up and not move quickly enough around the engine.

What Next – Your Car is running hot but the heater is blowing cold air?

A heater that only blows cold air can be tricky to diagnose. There are many different things that can affect the temperature of the air coming from the air vents, and many of them are hidden away behind the dashboard where they are difficult to examine.

  1. Begin by checking the fuses. Refer to the vehicle handbook for instructions on how to check for a blown fuse. The heater blower motor will have a dedicated fuse that may have blown, replacing the fuse may fix the problem.
  2. Check to see if the heater blower motor is working. If the fuse is not blown, check that the blower motor itself is working. Turn on the engine and set the air conditioning fan speed to its highest setting. You should be able to hear the fan running behind the dashboard. On some vehicles, you can get access to the blower fan by removing the pollen filter. If this is possible then you may be able to visually inspect the blower fan to see if it is actually working.
  3. Check the engine coolant level. Make sure that there is enough coolant in the engine so that the heater core can be adequately supplied when the engine is running and up to temperature. If the coolant level is low, top it up using recommended fluid and run the engine for a while until the thermostat opens and starts supplying the heater core.
  4. Check for collapsed or damaged coolant hoses. Check the rubber coolant hoses for damage such as splits or holes. Make sure that all of the hoses are fully expanded with no blockages. It can be difficult to check the heater core feeder hoses without dismantling the dashboard, so you may only be able to check the engine coolant hoses.
  5. Run a diagnostic of the ECU for trouble codes. If there is a problem with the blower fan, the engine thermostats, or the electric auxiliary fan then a trouble code may be recorded in the ECU. Common auxiliary fan codes include P0480, P0481, and P0695. Other cooling system codes that you may come across include P2181, P0115, P0116, and P0117.

Related Questions

Q1. Why does my heater blows cold air when my car is idling?

If you find that the heater starts blowing cold air when you are driving slowly or when idling, this may point to a problem with the water pump. On some cars, especially older models, the water pump will slow down when the car is stopped or driving slowly, like when stuck in traffic. This will slow the rate at which coolant is pumped to the heater core, causing the temperature of the air to drop.

If there is a blockage anywhere in the cooling system, air in the system or even low coolant, this can cause the heater to blow cold when idling.

Many newer cars are fitted with an auxiliary water pump that is powered electrically. This means that it will operate independently of engine RPM.

Q2. How do i know if my heater core is clogged?

Even though the heater core is hidden behind the dashboard and is difficult to see, there are some tell-tale signs of a clogged heater core. These include the heater not working properly, cold air blowing from the air vents, steamed-up windows, wet patches underneath the dashboard caused by coolant leaking from the heater core, and low engine coolant levels.

If the heater core is clogged this means that coolant is getting trapped in the heater core. A quick way to check if the coolant is getting in and out of the heater properly is to check the heater coolant hoses. If there is a blockage, the heater core feeder hose may feel more rigid and hotter than normal due to trapped coolant. The return hose may be cold and could also be collapsed if there is a blockage in the heater core that is causing an airlock.

Q3. Why would my car overheat but the heater blows cold air?

If your car is overheating but you are only getting cold air from the heater then this could be a sign of a blockage in the engine cooling system. If the coolant flow is restricted, this will cause the engine to run hot or overheat. It may also stop hot coolant from reaching the heater core and this will leave you with cold air coming from the vents.

The most common place in the cooling system for blockages to occur is in the radiator. The radiator can become blocked with dirt and rust over time and this impedes coolant flow and slows down its movement.

Another common cause of this problem is a faulty engine thermostat. Thermostats are designed in such a way that should they fail, they will not impede coolant flow. Sometimes the thermostat can stop working properly and can open and close at the wrong engine temperature. This can lead to the engine over heating and no coolant getting to the heater core.