Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Subarus and catalytic converters

We have a 2004 Subaru Forester and a 2007 Subaru Impreza. Within the last three years, we have had to replace the catalytic converter in each vehicle twice. We understand that this is very uncommon and are trying to figure out why we’re having such problems. Any insight? Might it be the garage where the work has been done? The vehicles are maintained (oil, etc.). We bought Subarus because we thought they were reliable, but are beginning to have our doubts.

How do you know that the catalytic converters REALLY had to be replaced?
Just because a particular mechanic told you so, that does not necessarily make it accurate.

Please give us particulars on what symptoms and/or trouble codes led to these CC replacements.

When a catalytic converter dies, it’s usually because some problem with the engine killed it. Why was the first one replaced? Was the underlying problem fixed at that time?

Also, is someone determining that the catalytic converter is actually bad? If they’re being replaced just because a particular code is set, then the proper diagnostic procedures aren’t being followed.

A lot more also has to be known about the vehicles also. All other things equal (and they never are), a vehicle that burns 1 quart of oil every 1,000 miles and travels 35,000 miles per year of mostly local driving in a cold climate can be expected to deposit far more carbon on the cat converter’s catalytic coating than a vehicle that burns 1 qt every 5,000 miles and travels 12,000 miles of mostly highway driving all in warm weather.

How many miles does each car have on it?
How much oil does each car use?
How many miles are put on each car?
What type of driving?
What climate?
Were the installed converters generics, “direct fit” aftermarket units, or OEM units?

It could have an underlying problem that was never addressed. Hate to suggest it…

I think it’s premature to blame the cars for this problem. You have not provided details about mileage, how the converters were diagnosed, and so on but it could be a misdiagnosis was involved.

If the converters actually failed then I agree with RemcoW that something else did them in.

The Impreza had 152,000 miles when the front catalytic converter was replaced last February - about 15,000 miles ago. The dealership and then our mechanic diagnosed problems with the O2 sensors and the front converter. Our mechanic did the repairs and got the part from the dealership. It turns out that the current problem resulted from the front converter sending its contents into the back converter which clogged the back converter and ruined both. (This time the car just stopped going and the check engine light was on, while the first time it was running but the check engine light was on.) We live in the northeast where the climate isn’t particularly severe and the car is driven mostly on the highway. The oil is changed every 3,000 miles and the car doesn’t seem to burn oil. Bottom line is that the front converter is under warranty, but now we also need to pay for the back converter, the towing charges, and labor - not to mention the hassle of not having a vehicle to get to work.

Appreciate the thoughts . . .

If you’ve disabled both the front and rear converters and you’re not using oil, you must be running rich. Very rich. I’d get this checked out.

Just in case the OP is not particularly automotively-oriented, mountainbike is referring to the engine taking in a fuel/air mixture that is too “rich” in fuel. That, and excessive oil burning, are the two prime causes of CC failure.

So…although these repairs are potentially costly, he was not referring to your economic status.
I just thought that I would clarify the terminology, in order to avoid confusion.


Good point, VDC.
“Running rich” in fuel creates a lot of carbon in the exhaust stream, and carbon deposits on the inner surfaces of the converter and destroys its ability to do its job. In severe cases it can even clog the honeycomb inside the converter and plug it up, which is what I suspect happened to your rear converter.