Subaru Catalytic converter

subaru
legacy
catalytic-converters

#1

If the dealer says I need to replace my catalytic converter is there something else wrong that with the car that will ruin the new converter?


#2

This is a 2002 Subaru Legacy with 112,000 miles on it. It seems to be running sluggish when accelerating but don’t Subarus always do that? I have only had it since February and have never had a Subaru myself.


#3

The likelihood is that there is a mechanical or electronic problem that caused the old CC to go bad, and that it will also take the new one with it.

Before you replace the CC, I strongly suggest that you have all of the services listed for 90k miles performed. Then, have a diagnostic run on the car, as well as a thorough overall examination of everything under the hood.

Hopefully, they will discover a problem such as a bad fuel pressure regulator or a leaking fuel injector, either of which can ruin a CC by allowing excess unburned fuel to enter the exhaust stream. Also, if the engine is burning an excessive amount of oil, that would be a likely cause of CC failure. I might add that if the engine is burning an excessive amount of oil at 112k miles, that is a pretty good indication that the previous owner(s) did not maintain it properly.

As to it being sluggish, that depends on what you are comparing it to.
If you previously had a 3,500 lb car with a 4 cylinder engine, it should accelerate just as well (or just as poorly) as that other car–assuming that both were properly maintained. I strongly suspect that this car–like most older used cars–got very little maintenance from its previous owner(s).


#4

Yes, there might be something else wrong that can ruin a converter. It’s also possible that the converter itself is fine and the downstream oxygen sensor is all that needs replacing.

The converter itself is primarily a ceranic honeycomb coated with platinum-palladium through which the exhaust is routed. When oxides of nitrogen (NOx) comes in contact with the heated rare metal on the honeycomb surface, the nitrogen and the oxygen is unbonded and becomes seperate atoms. The oxygen is picked up by passing carbon monoxide molecules and becomes CO2, the nitrogen flows through (air is 77% nitrogen).

The efficiency of the converter is monitored by placing an (“upstream”) oxygen sensor in front of the converter and one behind the converter (downstream) and comparing the signals. Oxygen sensors also work by their surface contact to the passing exhaust stream.

Things like burning oil and poor combustion that gives off carbon (soot) can coat the surface of the honeycomb, reducing the effectiveness of the converter. Those same things can also coat the surfaces of the oxygen sensors, affecting their accuracy…and leading to a false O2 signal that tells the ECU that the converter is bad when it’s actually the sensor that’s bad. The only ways to tell which is bad is by (1) actually looking at the O2 sensor readings on a scope or (2) changing the downstream oxygen sensor and seeing if the code disappears.

If the problem originated in the combustion chamber (burning oil or poor combustion) then the problem itself will need to be addressed. Unfortunately, the affected component will still need to be changed also.


#5

Why was your car brought to the dealer?

Did/do you have any CELs (Check Engine Light)?


#6

“Did/do you have any CELs (Check Engine Light)?”

Thank you for adding that question, Joseph.
I am beginning to lose patience with people who post questions, but only reveal the presence of the illuminated CEL when they are specifically questioned about it.

For some bizarre reason that I can’t fathom, all too many drivers seem to think that the CEL is irrelevant, to the point of driving with an illuminated CEL for…months…possibly years…but not mentioning this very relevant fact when posting a question.

So–is the CEL lit up?


#7

I agree, that was an excellent question.

When, oh when, will we advance to the point that a display screen posts a fault description directly to the driver instead of this simplistic Check Engine Light and follow-up code reading? Perhaps a direct message would at least emphasize the driver “hey, something is wrong here…see a mechanic”. Or am I being naieve? It’d save us that whole code reading exercise, too.

On the other hand, maybe there’s no CEL lit here…