Heater Core Problems - Breaking It Down

Recently, in our Ask a Mechanic section, user steprich asked about replacing the heater core on a 2002 Saturn L200:

Mechanics jerryh20, drivability, and the informative Big Block 409 all responded and got to the bottom of steprich's problem. You can read those exchanges here.

I thought this would be a good opportunity to talk about heater core problems in general which are are common topic of discussion on 10w40.com.

The Question

How much is it to replace the Heater Core on a 2002 Saturn L200? Liquid leaks into the car on the passenger side.

Breaking It Down: Heater Core Problems

Heater core problems and replacement questions are nothing new to the 10w40.com mechanic's forums. People with questions about heater cores in Plymouths, Oldsmobiles, Chevrolets, Fords, and every other make you can name. As vehicles age, heater cores are one of the more common problems - especially after a summer of disuse.

First, Diagnose the Problem

Before doing anything, the first thing you should do is determine whether or not it is, in fact, the heater core that is the problem. If so, you'll want to find out what the problem might actually be so you'll know whether it requires replacement or something else.

Leaking Liquid

If there is liquid leaking onto the floor underneath the dashboard, especially at center, then that's a good indication that the heater core is leaking. It's not a for-sure indication, though. Other problems like condensation, a leaking windshield, or even just spilled soda in the glove box are other things that can make liquid leak to the floor under the dash.

So if liquid is leaking, you'll need to get a sample of it to examine. If you can see where it's leaking to (a specific spot on the floor), you may be able to put a container underneath to catch some. Using a Q-tip, you can soak up some of the liquid as well.

What you're after is to see it untainted by background color (carpet, metal) so you can see if it's got the green tinge of anti-freeze or is just water.

If you cannot easily determine what type of liquid it is (i.e. it has no obvious green or yellow tint), then it could be something else. Use a garden hose on your windshield and then monitor for an increase in liquid leaking. Check the glove box to be sure there's nothing in there that could be leaking as well (you never know).

Finally, if you can garage the vehicle at all, do so in a heated space if at all possible. If there are no leaks the next day, your leak could actually be condensation.

If you're absolutely sure it's the heater core or if your problem is not a leak, then look for other common problems before committing to taking apart your dashboard to gain access to the heater core.

Heater Does Not Turn On

If you turn on the heat on the climate controls and nothing happens, you could have a defective heater core. Or defective climate controls or problems with the louvers that control air flow through the heater core and into the cabin.

Before disassembling the dashboard to replace the core, check that your climate controls are working. Using the climate switches, determine if you get air conditioning (if equipped) into the cabin when set to do so. If you do not have A/C or if it is performing correctly, then try feeling for heat.

Most vehicles have an access panel at the center or just to the right of center (passenger's side) that allows limited access into the dashboard from below. This usually exposes one or both sides of the heater core or at least its containment area in the duct work.

With the engine at temperature and running, turn on the heat via the cabin controls and then carefully feel inside that area to determine if there is heat coming from the core. If not, it may be clogged. If so, your problem may be your controls rather than the core itself.

You may be able to see the louvers that control air flow through the core. If so, make sure they open when you open up cabin air to heat on the climate controls.

Replacing the Heater Core

You may be tempted to buy a "heater core repair kit" from the auto parts store. Most mechanics will tell you to shy away from these as they rarely work and when they do, it's usually only temporary. Further, bypassing the core by splicing the two coolant hoses that connect to it is a safety concern because the heater core provides the warm air to defrost windows.

So replacement is your best option. It's a big job, though, requiring a relatively thorough disassemble of the dashboard. This can take hours.

Since every vehicle is different, some have better or easier access than do others and most will have hidden clips, screws, and other items holding dashboard pieces in place. Refer to a good repair manual or shop manual for your specific vehicle for details.

Two hoses will be attached to the heater core, which itself looks like a small version of your engine's radiator. One is the "inlet" and the other is the "outlet" - be sure to mark them before removing them from the core. If your heater core is leaking, it is almost always at these hose connection points, so check for leaks before removal and you may save having to buy a new core for replacement.

Re-attach the hoses in the same way they were removed (most mechanics will trim a bit off the ends of the hoses to keep them from cracking) and then re-assemble everything in reverse order.

Most DIY mechanics can do this job in a weekend. A good mechanic will take 3-8 hours, depending on the vehicle.