Car Heater Not Hot

Reader Question:

The heater in my car is not working. The mechanic told me my heater core/coil is leaking antifreeze and needs to be replaced. The price he quoted me for a heater core replacement is not in my budget right now, do I have another option?



Hi there Bill

I feel for you, the heater in your car (like the air conditioner) is a wonderful comfort item, and it can be very costly to repair if and when it fails. Before we get into repairing it, let me explain briefly what the heater core (picture) does.

The heater core works in conjunction with your engine’s cooling system. The function of the cooling system is to remove heat from the engine, and it does this for the most part by sending the heated anti-freeze to the radiator located in the front of the car.

The position of the radiator allows outside air to blow across the radiator thus cooling the anti-freeze.

The anti-freeze is then sent back to the engine. Hot anti-freeze is circulated throughout the cooling system by the radiator and heater hoses. Think of the heater core as a small radiator located inside the dashboard of your vehicle.

Anti-freeze is constantly being circulated throughout the engine, radiator, and yes even the heater core regardless of whether you have the heater dashboard switch on.
When the heater is turned on by the driver, a diverter door opens to the heater core area in the dashboard. A small fan (commonly referred to as the a/c fan or blower) blows air across the hot heater core into the duct work of the dashboard and into the interior of your car.

When the heater core leaks (it leaks anti-freeze of course) it will usually leak inside the car on the passenger floorboard under the carpet. If your heater isn’t working properly, or if you smell a sweet odor, investigate the passenger-side floorboard for signs of anti-freeze leakage.

A leaking heater core may also cause a greasy film on the inside of the windows.

So what causes the heater core to leak in the first place? Usually the main culprit is rust build up caused from lack of cooling system flushes. Anti-freeze acts as a lubricant and rust inhibitor as well as a temperature controlling substance.

Anti-freeze should be flushed and replaced periodically to keep the cooling system in good working order.

Rusty anti-freeze is usually a sign of a coolant leak somewhere in the cooling system that has allowed air to enter the system. It may be a leaky radiator hose, water pump, heater core, radiator, etc. Rust build up can be just as damaging to the radiator and other internal engine parts.

How do you stop the anti-freeze from leaking onto the floor board? Obviously you can replace the leaking heater core itself, which is the correct fix and is what your mechanic recommended.

You can also try a can of radiator stop leak additive to see if it will patch the hole (this might be an acceptable temporary repair), or you can cut off the flow of anti-freeze to the heater core all together.

By-Passing a Leaking Heater Core

There are two heater hoses that are attached to the heater core from under the hood. Click for picture. These hoses can be cut and blocked off with a hose clamp, or a small hose splice can be inserted between the two hoses to create a loop thus avoiding the heater core all together. This works great to temporarily repair a leaking heater in the summer time.

Another cause of a heater that is not leaking but is not heating properly can be a faulty thermostat. The thermostat is calibrated to keep the anti-freeze inside the engine at a constant temperature. A faulty thermostat might not allow the engine temperature to get hot enough to heat the anti-freeze.

Thermostats generally do not need periodic replacement unless there is an under- or over-heating situation. When replacing the thermostat, make sure to install the proper heat range recommended by the manufacturer.

Proper engine temperature plays a vital role in fuel economy and overall running condition of the engine, as well as regulating cooling system temperature.

Low anti-freeze levels or poor circulation of anti-freeze throughout the cooling system will hamper heater performance as well. If there is not enough hot anti-freeze to circulate and deliver to the heater core, heater efficiency will be greatly reduced.

If you suspect a problem with your heater, first check the anti-freeze level in the radiator and make sure the fluid is in good shape and of proper color (either green or orange/red if you’re using one of the new extended life products on the market).

To check anti-freeze color, dip some out and look at it in a glass container. (We use an anti-freeze hydrometer, which is basically an expensive turkey baster.) When you are looking at the anti-freeze in the radiator, you can only see the top surface color, and it will usually look okay even if it’s not.

If low or contaminated fluid is not the problem, feel the two heater hoses going from the engine to the heater to make sure they are hot (the engine needs to be at normal operating temperature). If the heater hoses are not hot, inspect the radiator hoses for internal cooling system blockage.

With the engine at normal operating temperature, the upper radiator hose should be very hot, and the lower radiator hose should be just slightly less hot.

If there is a substantial temperature variation, a blockage or restriction is probably present. A faulty thermostat and a radiator or heater core that is full of calcium deposits or rust build up are a few causes of cooling system restrictions.

Austin Davis

Posted in: Coolant Leak

About the Author:

Austin Davis, consumer car repair advocate. "Hi there! I love to help people solve their car repair problems and I hope my site was helpful to you today. Thank you for stopping by."
  • Judy DP

    hello, Mr. Austin
    I have been driving for over a year now on what might be a bad heater core. However, only once or twice has the heat not worked, and the floor has never been wet. There is usually a faint antifreeze smell, and on very few occasions was bad enough that I had to ride with the windows down, from it being overpowering. I did have a shop put some pressure on the cooling system (I think that’s what it was), they said there was no leaks. That very day, when I drove it home, I couldn’t hardly drive from the amount of fogging the windows were doing. They don’t usually fog up anymore, and it is very sporadic when they do. Now, mainly its when it is raining out that I have the fogging up, and it is kinda filmy, but not that bad. One day, when it was raining out, it was so bad that it actually looked like smoke billowing out of the cab of the car, when I would open the door. But that was a one time incident. Also, there is usually a ‘thud’ kinda noise coming from up under the dash right after I start the car. Is it possible that like my heater valve switch or some kind of switch inside the dashboard is causing all of this. I don’t know jack about mechanics, but seems if it was a bad heater core, and for this long, that the floorboard would be getting wet and the heater would be not working very well. Oh yeah, also when I have the fan blower switch off, there is still some air coming out of the vents, whether it is from the air conditioning or heater setting. And when it was on the air conditioning setting but the fan turned off the air coming out was pretty warm. Any help would be greatly appreciated

  • Austin Davis

    Thank you so much for your comment Johnn, let me know if you need anything else…just use the comment feature and I will reply.

  • Johnn Perdoncin

    I am not an expert in cars and my knowledge in mechanics is very limited. But after reading the article and reading the comments I gotta say your explanation is really good. My heater wasn’t working and I couldn’t figure out why, I had no money to take for a test so when I read that the the heater core leaks I went to check the passenger side and there it was! I never smelled anything but the leak was there, after knowing this I took it to my mechanic knowing what the problem was and the repair was not that expensive. Thanks a lot for the advice!

  • Austin Davis

    Great News Jerry, thanks for updating us. Glad the new thermostat helped with the heating issue.

    The water leak problem is pretty good news that it is CLEAR water and not coolant…which should be a cheaper and easier repair.

    Make sure the A/C drain is clean. There should be a small drain hole/tube under the passenger side of the vehicle and it can sometimes get clogged up with leaves and debris and when you run the A/C the condensate water can not drain out of the evaporator, and lands up on the floor inside the vehicle.

    I like to take compressed air and shoot it up the drain hose from under the vehicle to help clear out the leaves.

    Also make sure the windshield gasket is in good shape and not leaking rain water inside the vehicle as well.

    Good job Jerry, thanks for the comment and the bookmark. :)

  • Jerry

    Okay Austin here is where I’m at.

    I had some other things pop up and wasn’t able to get back at it or to you as soon as I would have liked to.

    The pressure test was weird, Grand Am system is not what I’m used to, with a radiator cap in the most logical place, ON TOP OF THE RADIATOR, and a overflow reservoir off to the side. This is like a sealed system, there is not a cap on the radiator. The cap on the reservoir is the one and only cap in the system. I put 15lbs. pressure on the system and it held just fine. Pressure tested the cap separately and it didn’t hold, however after going to the parts store and getting a new cap I tested the new one and had the same results, I did try different adapters from the pressure kit on both, I don’t know what that was about. I put the new one on anyway.

    I took the car for a 25 mile ride (again my daughters car I don’t drive it often) It was 20 degrees out and I froze my butt off, got just a little heat but not much at all, figured thermostat is stuck open. Bought one but then had to stop for a few days, left the car in the garage out of the elements. Just finished with thermostat today, another fun filled thing to do on this car. Heat is fabulous, system pressures up fine, temp gauge on dash goes to approx 170-180 it’s all great but….

    You knew there was going to be one didn’t you? I thought I had the floor completely dry, with the hair dryer while doing the thermostat, buttoned it up took it for a ride, got back felt the floor and one place on the drivers side, closer to the seat on the door side was still pretty moist. I’m hoping that maybe I just got the surface dry and after sitting for awhile and possibly me stepping on it, that the padding was still wet and it wicked up to the carpet. It is definitely water not antifreeze thank God.

    I did try to pull the carpet & padding back however the rocker panel plastic trim cover holding the carpet all along the door side is almost impossible to pull up because the 18yr. old screws in it are rusted and not coming out unless I drill them out. I did see that there might be an issue with the windshield on the drivers side, I will make sure the carpet is absolutely dry then wet down the windshield and see what happens and go from there.

    Again thanks for the site, this is great!!! I’ve bookmarked it and I am sure I will visit often.

  • Austin Davis

    Thank you so much Jerry!

    Well, first thing you need to do is get a cooling system pressure test to help determine where (if any) the leak is coming from.

    I guess you could have a defective new core, but doubt it. ALTHOUGH a few years back A/C Delco did have a rash of defective heater cores, mainly in their truck models.

    You can take a pair of vice grip pliers and block off both heater hoses and see if the system will hold pressure. it wont hurt anything to do that…seen customers that have done that for years when I pop open their hood and see the pliers I have to laugh.

    If you have coolant in the system I would expect that wetness on the carpet to feel greasy to your touch. If not, and she has been running the A/C compressor the wetness could be due to a restricted A/C drain…water should be clear on floor and NOT greasy. The a/c drain usually leaks more on the driver side than the passenger…and heater core usually just on the passenger side.

    Also, don’t rule out a water leak around a windshield gasket, seen that many times in older vehicles. Get someone to wash down the windshield area as you are up under the dashboard. Course, that would not effect the cooling system or lack of pressure on the heater hoses.

    The water is UNDER the carpet, so you will need to pull back the carpet some and get the “jute” material under the carpet dry. A hair dryer will come in handy.

    As for the lack of temp gauge movement, I would try replacing the thermostat if you have determined with the pressure test there are no leaks. Also, make darn sure you have bled all air out of the system, which can be a pain sometimes.

    let me know how things work out

  • Jerry

    Hello Austin,

    I have to tell you that this is the most thorough, complete and easily understood explanation of a heater core I have seen. I should know because I believe I have read everything out there on the subject in the past few days. Ha Ha!

    Here is my tale of woe. A week or so ago my 18yr old daughter calls and says her cars dash light says “Low Coolant Level” (1995 Pontiac Grand Am 3.1L). I ask if it is overheating or if the temp gauge is reading high. The answer is no. I tell her to come home and park in the drive. As I walk out to it there is a little stream of liquid coming out underneath. I pop the hood, no steam or anything obvious. The radiator hoses are soft with no pressure and I am able to open the cap with no problem, very odd. She had been driving for roughly 25 miles.

    A little history.
    A few weeks prior I had gone out to her car one very cold morning, I saw a fast food drink cup laying on the passenger side floor, I went to pick it up and found that the floor was extremely wet. As any good dad would do?… I went into the house and gave her hell for being a slob and spilling a full cup of pop and not even bothering to clean it up or anything. As luck would have it, over the next few days she tells me that she has to scrape the insides of her windows because they are icing up. Again being the know-it-all idiot that I am, I proceed to explain to her the moisture from the pop is causing this and she should have cleaned it up yada, yada!!
    Back to the present.

    As I’m looking inside I notice how soaked the front carpet is on both sides. Then the light bulb in my head goes on, HEATER CORE!!! I immediately and repeatedly have apologized, that hurt, Ha Ha!

    Having replaced some in the past I do my research and dive in. What a pain in the rump, but I finally get it done. I use a shop vac to vacuum as much out of the carpet as I can. I put newspaper down to help absorb some as well.
    I put it all back together, add antifreeze, start it, bleed it, and let it run for awhile. Seems good except for the fact that the Temp gauge only moves a little, still no pressure on radiator hoses and I can take the cap off no problem. Also I haven’t seen or heard the engine fan go on. A couple days go by, I have been letting her use my car in the meantime. It seems like the carpet on the drivers side is as wet as it was a few days ago.

    The heater hoses that connect to the core from the engine compartment seemed pliable, very hard to see mostly done by feel. I guess it could be a faulty new core. I wasn’t able to and didn’t check before putting it all back together. Because I had to pull the console and a few dash panels and such to get at the core. I am at my wits end. If I can pull back some of the carpet and pad will I be able to see, or figure out where the leak is coming from?

    Any and all thoughts are welcomed with open arms! Thanks for any help.

    Jerry Pro