Can a bad catalytic converter cause O2 sensor issues

I am developing a love hate relationship with my car, 2001 sub forrester. In 2009 it was intermittently stuttering: tuneup, new coil, finally O2 sensor (both) replaced which fixed it. In Oct started to do the same thing. First the back then the front sensor replaced but they commented the flow was low on the catalytic converter but wouldn’t say it could be the cause of the sensor issue. Now the O2 sensors are on either end of the coverter and the car is starting with early signs already. Is is reasonable that it is the catalytic converter? I am wrestling with just having them do the full tuneup and replace the converter or am I crazy? It will be $$.

No, a bad converter couldn’t harm an O2 sensor.
OTOH a bad O2 sensor can be mistaken for a bad converter.

I think there’s some other underlying problem here.
Considering it’s a Subaru my first guess would be head gasket.
Coolant leaking into the cylinders can ruin an O2 sensor.

My daughters Forester kept complaining about O2 issues as well a while back. She brought it to her dealer and they suggested a new CAT and both O2 sensors, quoting a crazy bill - I told her to not bring it there but now she knows.

After measuring the levels coming off, it was clear they were in range of normal but one sensor’s response was slow.
The ECM calculates the catalytic converter’s efficiency by making a ratio of the front O2 sensor over the rear sensor. If it exceeds some number, it throws a code. Since one of them was slower in response, the readings would be lagged and the ECM was fooled into thinking that the ratio was off when it actually wasn’t.
Apparently is it a common complaint of Subarus of that vintage.

I replaced the sensor and the problem went away to come back 6 months later. Same issue but this time its emission’s test was due. I reset the code and got the emissions tested. It passed with flying colors. In CT they’ll test emissions, even with the computer reset, by putting a probe in the tailpipe. As long as the light is not on, it can pass emissions.
Clearly the CAT is fine, if it can pass emissions.

Getting sick of that code popping up periodically and masking other possibly more serious problems, I got two spark plug extenders at pepboys, drilling one out. I then took the rear O2 sensor out, put the drilled out plug extender in its place, threaded the drilled out one on top and put the O2 sensor into that one.
The sensor now sees less O2 so the ratio is fine, as far as the ECM is concerned, even with the reading lag.

Long story short:
Is it a hack? Sure. But one can’t keep buying O2 sensors and cat converters.

Can your problem be related to a bad cat or O2 sensors? Sure. I do think we replace catalytic converters and O2 far too quickly. For something that supposed to enhance the environment by causing less pollution, we replace them with things that create a lot more pollution in its manufacture. Seems backwards to me.
Bring it to another mechanic you trust and see what they say. Many mechanics will just replace the entire system because they want that light to go off and not see you come back. That often costs money.

You now have two spark-plug extenders stacked into each other with the sensor at the top of the stack, virtually out of the exhaust flow??

They still make “spark plug extenders”?? Maybe they have found a new market for them!

Yup! I was pleasantly surprised that the thread on them was identical to the O2 sensor.

Note, this is not something I’d recommend doing when you get O2 sensor related errors but it works as a good last resort.
The one behind the CAT is really just there to annoy you by turning the light on. It doesn’t make any adjustments to the engine based on what it reads. Since the emissions are perfectly okay, I don’t feel bad cheating the system.
For new soobies they actually reflash the ECM. I suspect the firmware has some sort of bug that does not allow for fluctuations in reading due to sensor aging so they fixed it.

Remco is exactly correct in describing the limited function of the downstream O2 sensor(s), however allowing the light to stay lit and ignoring it elimiinates the ability of the OBDII system to tell you should you develop eny other problems…and you inevitably will on a 10 year old vehicle. It’s like disabling the smoke alarm permanently because every time you burn popcorn it goes off.

Cat converters have no moving parts. And the term “catalyst” means by definition that it cause change without itself changing. Converters don’t “go bad”. Some other problem poisons them. The same two problems that generally do this also poison oxygen sensors on the same way, by coatting the surface. Those two common problems are (1) burning oil and (2) burning coolant.

Circuitsmith is correct. There’s an underlying problem here.

How many miles does your Subie have?
How much oil are you using?
Are you losing any coolant? How much?

Thanks for the feedback!!! The car has 116,000 mi on it. Had the head gasket replace several years ago. Oil level is at “add” haven’t added anything since June and coolant shows full but when look in the radiator can’t see any. Had the code checked at subaru & they say evac leak. Smoke test in October when this started the first time and showed similar code. Fixed that no change in how the car ran.

The missing fluid in the radiator is not good. The way the system should work is that as the coolant expands with heat beyond the pressure that the radiator cap is speced at (tyically 15-16 psi), the coolant pushes past the cap and into the reservoir. Then, when the engine’s coolant contracts as it cools, it draws coolant back past the cap and back into the radiator, keeping the engine and radiator always full.

I think what might be happening is that a breech in your headgaskey may be allowing hot combustion gasses to blow into your water jacket. That’s blowing coolant out the engine and radiator and into the reservoir, which is then allowing it to blow out the overflow tube as you drive. The reason you’re not seeing excess temps on your temp gage may be that the fluid has dropped too low and the gage is measuring air instead of coolant.

That scenerio would answer the cooant level questions as well as the rough operation and the cat converter and/or oxygen senaor codes, as coolant would also be getting drawn into the combustion chamber and burned, coating the surfaces of both.

I think the place to start here is with a compression leakdown test. If you have a breeched headgasket, no fix will take until you coorrect that.

OK4450 is an expert in these matters. Perhaps he could offer his insight as well.