Once you’ve found out that you have this kind of problem the next logical question is how to fix a heater core leak.

There are various solutions, some of them are relatively simple to apply while others are more complicated.

Some you can do on your own while for others, you may need professional help.

This article should help get a grasp on the whole topic and after that make a good choice of what to do.

If you’re not sure if the heater core is actually causing the problem, you can read a separate article about the most common symptoms of a bad heater core by clicking here.



This solution for a heater core leak is perhaps the best but also the most complicated.

The main reason for this is the location of the heater core. On most cars, it’s located way beneath the dashboard, between the lower part of the dash and the firewall.

It’s buried under various plastic panels, assemblies, switches, and cables for controlling the heating and cooling and dozens of other screws, bolts, nuts, and else.

Reaching it is a nightmare on most cars and means a whole day of work (even a couple of days) under the circumstance that everything goes well and nothing breaks.


After that, returning everything and more importantly making it all work properly is a matriculate and painstaking job.

Don’t get me wrong, it can all be done and in quality, it just takes time and a lot of patience. Besides this, if you drive the car to a mechanic, it will also be expensive.

  • Reasons for heater core problems

Just like with the radiator (and the heater core is essentially a small radiator), the main reasons for problems are material fatigue.

Coolant is a pretty aggressive substance and this combined with constant heat takes its toll over time.

One more crucial reason is that the heater core isn’t used all the time. For long periods (during the whole summer) it lays dormant filled with leftover coolant. This can cause clogs but even worse, corrosion which inevitably leads to leaks, both from the core and the exit pipes.

After years of exploitation, despite the fact that it’s tucked safely inside the passenger cabin, it gives in. It takes years, even decades, but it does eventually happen.

Replacing the heater core itself is not complicated at all once you reach it. In most cases, it’s held in place by a couple of screws or a plastic assembly. Once you’ve removed that, two clamps need to be released in order to separate the coolant hoses.

When returning the new heater core, it’s mounted back into the assembly and the hoses are put back and tightened.

This is the common scenario in most cars.

To simplify, replacing the heater is pretty easy. Getting to it is the hard part, very hard part.

That’s why people, before replacing the heater core, try some of the next steps.




Stop leak is a very popular remedy when talking about how to fix a heater core leak.

It’s simple to apply and if successful, the fix is over in a matter of minutes.

Stop leak is applied like this: you open the coolant tank, start the car and pour the liquid into the coolant system.

After that, the engine is left to run for some time.

Because of the coolant circulation, the particles from the stop leak liquid go around the system and „find“ where the puncture or damaged place is.

Then, it starts forming a layer over that place and eventually stops the leak. It puts a plug on the hole, so to say.

Again, this is the standard scenario, more information on how each and every stop leak works is best found on the bottle or on the manufacturer’s website.

If it works, the heater core leak should stop in a matter of minutes.

The downside of stop leak (at least in my opinion) is that it helps only if the puncture or damage is small and that it has a limited duration.

With every flush of the coolant system, the effect will probably disappear.

All in all, a good, short-term solution until you find a better one. It’s not even that short term, some fixes can last for months, but as opposed to replacing or fixing the heater core, it certainly is.




One more solution for how to fix a heater core leak that is immensely popular although not a quality one.

Blocking the heater core means separating it completely from the rest of the coolant system.

The heater core is connected by two hoses. Through the first, coolant gets into the heater while through the other it returns back.

Blocking the heater core means connecting these two hoses to each other using a plastic (or metal) „U“ shaped pipe.

This is better known as bypassing the heater core.

The end result is a closed-loop in the coolant system, leaving the heater core dry and without coolant flow.

No flow, no leak.

Why is this a low-quality solution? Well, the answer is obvious. Everything is OK until the winter comes when you’re completely leashed of heating rendering the car pretty much useless.

So, rather an improvisation than a real solution and fix.

Nevertheless, in case of an emergency or the lack of a new heater core, you should keep this in mind.

It’s also cheap, all you need is two clamps and a piece of „U“ shaped plastic or metal pipe. A couple of Euros or Dollars at most.

If you want to learn more about the heater core, click here for a great explanation from Wikipedia.


Besides inspecting the heater core itself, also check everything in its vicinity. Namely, the control valve, hoses, connections as well as clamps.

These reasons are a more common source of problems, some of these can even mimic an actual heater core problem.

Always double-check these before you start any huge dismantling and replacing parts.

  • Heater core control valve


On most systems, there is a control valve for regulating the coolant flow through the heater core.

Although in some cases it isn’t integrated with it but is a separate part, it’s definitely worth the check.

The most common issue is the valve release mechanism which gives in, mostly due to material fatigue.

Leaks first start with a couple of drops but in the final phase, it can become a stream.

It’s definitely one of the first places you should check when you have a coolant leak.

Control valves are opened and closed by the temperature command on the dashboard.

When the valve is the problem, the best and sometimes only solution is a replacement. It involves taking off two hoses with clamps and a control cable.

Pretty simple but the bigger problem is accessibility. It all depends on the location of the control valve.

Sometimes it can be done without much dismantling of the dashboard and surroundings, on some, it’s pretty much the same job as changing the heater core.

  • Coolant hoses, connections, and clamps


The solution for this problem is simple provided you can find and reach the problematic spot without much dismantling.

If you’re lucky, a punctured hose, a loose or broken clamp, various connection pipes, can be easily replaced.




The last and probably least used option is to fix the old heater core.

When mentioning how to fix a heater core leak, people usually avoid this because the price of a new one is similar or even less than mending the old one.

Besides this, in most cases, repairing the old heater core is never as good as installing a new one. Also, there’s always the risk of a new leak appearing very soon.

No one wants this especially after doing a huge, complicated, and time-consuming job of dismantling to reach it.

But when the situation demands it (lack of time, lack of spare parts or you have an old-timer for instance), it is a valid option.

The main issue is to find a good repair shop that will do it in quality. If you can’t find a specialized shop for this (and they are rare), then search for a company that repairs climate systems. They are most likely to have the tools and know-how to help you.

When there’s only a small puncture, some quality welding or soldering (depending on the material of the heater core) may solve the problem pretty well.

On the other hand, avoid this if the heater core is seriously damaged, like from scale build-up and corrosion. Even if the mending is successful, it will be short-lived for sure.

So, find a good shop and ask, if possible, for an assessment of the repair.

But even if it can be repaired, see that you buy a new heater core as soon as possible, just in case.



Hopefully, after reading this article you’ll get a better picture of how to fix a heater core leak.

Once again, if you see that the heater itself is a problem, best replace it with a new one and call it a day.

Improvisations are OK (if done properly)but know that most of them are more or less, short-lived. Some will last longer, some shorter, but can’t match replacing with a new one.

Also, an improvisation combined with a huge dismantling job is a risky choice. Doing all that work only to have the same problem again in a couple of months is not a good option.

As mentioned, check everything around the heater core thoroughly before you start dismantling, maybe something else is the problem. The last thing you’d want is to pull part everything only to conclude that the problem was a simple clamp or hose.

For the end and perhaps most important of all: tend to a heater core leak as soon as possible. It will lead to a minor loss of coolant at first but it will get worse over time.

Loss of coolant inevitably means engine overheating and this can lead to engine damage or even a partial engine overhaul.


Written by: Sibin Spasojevic


Former car technician, life-long car and DIY enthusiast, author for

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