Car Heater Troubleshooting

Car Heater Troubleshooting Steps:


It is coming to be that time of year when the weather gets cold and we need to use our car’s heater functions. The heating system in a car is a very simple setup once you take a minute to observe it’s operation and understand the theory of how it works, and since it is an easy system it is usually fairly easy to diagnose a heater concern.


Today we will be learning how to find the problem when the car does not have sufficient heat when the heater is turned on. I am going to list the steps below in which I find it easiest to diagnose a no heat issue, everybody will have their own process, so I hope you find this useful!


Step One:

Make sure the engine coolant level is full! I cannot stress this enough. Over half of the no heat issues I have repaired were due to a low coolant level. The heater works by running hot coolant through the heater core assembly, which then heats the air in the HVAC housing and changes the air outlet temperature. If the coolant is low it will not have enough pressure to push up and through the heater core, which results in no hot air. The coolant should be visible in the reservoir, if the coolant looks low/empty then you need to check for leaks, if there are leaks they will usually be visible by inspecting the cooling system hoses/seals/pump/radiator etc. If you do not see a leak then the next step is using a Coolant pressure tester. If the coolant level is good then proceed to step two.



Coolant minimum/maximum capacity level indicator


Step Two:

Check the heater core inlet/outlet temperature. If the heater core has hot coolant going in, and no heat coming out that tells us that the heater core is plugged, and not allowing proper temperature to be achieved. If you have one hot hose going in and one cool hose then you will need to attempt to get the heater core flushed/unplugged, some have success with this, others do not, I have always preferred to put in a new core, but certain vehicles that can be a pain in the ass, so attempting to flush it may be the way to go if you want to try and save yourself a headache. If the inlet/outlet hoses are both hot then the problem is not in the heater core. The easiest way to test the inlet/outlet temperature is by hand, but if you are not sure then you can use a digital Thermometer to inspect the temperature at both hoses. If the temperature at both hoses is correct then proceed to step three.


Flushing a Heater Core


Step Three:

If the cooling system all tests out good then that leads the problem to the interior temperature blend mechanism. The blend door is what controls the temperature output at the vents. The door is moved by an actuator assembly that is controlled by the temperature control switch on the dash. If there is heat coming through the heater core and the vents still will not blow hot air then that means there is an issue with the temperature blend system. Sometimes these will set a trouble code, but most of the time it is something you manually have to verify and diagnose. The majority of the time it is a bad actuator, but it is possible to have a broke/bound up blend door, I have seen some weird things fall into the vents and cause blockages as well.

HVAC Housing, contains blend doors and vents


These are the basic steps to check for diagnosing a heater issue. If you verify all of the above is still operating and you have heater issues then it may be a bit harder problem to which you will want to know what you are doing before tackling it. If you need some personal troubleshooting help for a tricky issue let me know, I love to help solve the weird problems!


Stay safe and have fun!


  • David says:

    Hi Gabriel,

    Thanks for this.

    The image of the engine coolant seems to show that the coolant in that image is above the maximum.

    I would have thought, not being a car mechanic, that the coolant level should be between the max and the min points, otherwise, why display the max and the min points.

    In the case of that image, as it is above the max, is that actually all right? I thought, not being a car mechanic, that if you put in too much coolant that that would result in problems down the road. Pun not intended. 🙂

    Kind Regards,

    • GGrey says:

      Hello David, thanks for stopping by!!

      You have a good eye, you are correct that the picture is shown above the max line, if it is over by that much it will be ok. What happens is over time when you are driving the engine will get hot and expand a bit, when this happens the excess coolant in the engine will vent back into the tank, which causes it to look a bit overfull. As long as the tank is not totally full it will be fine 🙂

      Take care,


  • eric says:

    Hello Gabriel, I was reading your article about having the check engine light on, I agree that an OBD tool would benefit a lot of people for working on their own car for some of the easier problems they might have with their vehicle.

    If someone doesn’t have the money to buy a scanner, a lot of the parts stores out there will connect one to your car and read the codes for you too.

    • GGrey says:

      Hey Eric, you are absolutely correct! Most major parts stores are able to scan and delete codes for you now, this can be very helpful for those who cannot afford their own scanner!!

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