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Guest Message by DevFuse

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Threaded steel "insert" tore off catalyst when replacing O2 sensor - help!

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9 replies to this topic

#1 harborseal55


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Posted 24 May 2004 - 11:12 AM

Oh oh..I was in the process of changing the rear oxygen sensor in the catalytic converter on my 96 Legacy 2.2 (170K miles) and - eek! - the thick threaded steel section (the section that holds the female threads that the O2 sensor screws into) that is welded onto the sheet metal catalyst body tore out of the catalyst body! I had followed the instructions in the Haynes manual that recommended warming up the car to heat and expand the catalyst to facilitate easier removal of the O2 sensor, and I don't believe that it loosened at all, the entire O2 sensor/thread assemble just snapped out. Did I do this incorrectly or do O2 sensors often get frozen into the catalysts? Thank goodness I just passed California smog inspection last January.

A new catalyst from Subaru is really expensive, so what are my options:

(1) Buy a new catalyst from Subaru?

(2) Would a welding or muffler shop be willing to weld the threaded insert back onto my old catalytic converter, or are shops unwilling to repair an old catalyst?

(3) Is there a threaded insert that I can buy to fix this (if this seems to not be an unusual problem when replacing O2 sensors)

(4) If there are no other options, where else can I go to get a replacement catalyst other then the dealer? So far I haven't found anyone who can even special order a replacement catalyst for me (one that uses two O2 sensors).

Any other options?

In the mean time, I plugged the hole in the catalyst by using a 1/2-inch national pipe thread (NPT) tap to apply a couple of threads in the catalyst and inserted a 1/2-inch pipe "plug," and unplugged the old (rear) O2 sensor from the harness and removed it completely. The car runs fine (as good as it did prior to my attempted fix), and the OBD-II scanner I have did not reveal any new codes as a result of the sensor removal (other than the previous P0420 catalyst efficiency code which was the reason for replacing the O2 sensor to begin with). As far as I can tell, the OBD-II system doesn't seem to mind that the sensor is gone, but this is a short-term fix to make the car driveable, I do want to pursue a permanent fix.

#2 Setright


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Posted 24 May 2004 - 12:53 PM

If the cat is very old and rusted, a blacksmith will probably not want to start welding on it, cause it won't last. However, you might be able to get someone to make a new, cat-less, pipe, that has two threaded inserts for the O2 sensors. Yes, it may not be legal, but think of all the good you will do for the environment in terms of fuel economy improvement. Expect 5-10%.

Or buy a new Sube cat. The money you might be able to save by buying a non-OE cat will be offset by the frustration of discovering that the cheap part doesn't fit right.
Get new gaskets for all the sections you intend to disconnect.

#3 NOMAD327


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Posted 24 May 2004 - 12:55 PM

I believe these are a straight or bolt type thread, If you have clearance to get in with a weld torch, you should be able to weld a new steel thread insert to the catalyst body. I would start with a nut that threads onto the new sensor and cut it down to about the thickness of the broken off insert. If it's a bit bigger around than the old insert, that may cover up any damage to the shell. If need be, a flat patch could be applied and a new spot picked to weld on the nut. The internals are meant for high heat, but I would weld a bit, and then let it cool awhile before proceeding to avoid melting the beads. I will guess that the thread is about an 18 mm, but they may vary from brand to brand.

#4 99obw


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Posted 24 May 2004 - 12:56 PM

(3) Yes. It's called a bung.

#5 Nug


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Posted 24 May 2004 - 02:58 PM

You can get them from Summit. Your local speed shop should be able to get them, also. Common thing to need when adding F.I. to a car that didn't have it before.

#6 JOEK39


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Posted 24 May 2004 - 09:33 PM

You can actually drive without the rear O2 sensor, and IF EVERYTHING ELSE IS WORKING PROPERLY YOU WILL SEE LITTLE IF ANY DIFFERENCE.

The FRONT O2 sensor is the primary instrument that the computer system uses to constantly adjust the fuel mixture of the engine.

The REAR O2 sensor is used to monitor the performance of the engine, and is the primary monitor for the performance of the catalytic converter. If the front sensor, catalytic converter, or possibly other items were to become worn out or miscalibrated, but still operable (but perhaps short of failure), the rear O2 sensor would sense an incorrect fuel mixture and signal a failure code in the OBDII system.

Again, you can drive it, but will be lacking amn important monitor of engine component miscalibration/failure.

Personally, I would get it fixed soon, but I wouldn't lose any sleep over it - don't sweat it, and I would go for the lower $$ cost solutions if at all possible.

Good Luck,

#7 JOEK39


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Posted 24 May 2004 - 09:35 PM

Rereading your oringinal post, your OBDII error code very possibly is indicating your catalytic converter is not operating as designed. The rear O2 sensor itself may be fine.

Let us know, know I'm really curious! :grin:

#8 hklaine


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Posted 24 May 2004 - 10:59 PM

What did you pay for your O2 sensors and where did you get them from?

I am doing my 90k next weekend and just today was considering replacing the O2 sensors as maintenance. Necessary?

How long does the manual suggest warming up the engine for before attempting removal?

As for your sensor, if the metal around the damage is in good shape I would weld (or find somene to weld) the mount back to the exhaust or mount a new one (perhaps in a different location) as NOMAD suggested. But before doing any of that, looking into the exhaust for a problem that might have triggered the error is certainly a good suggestion.

Do any manuals have tolerances for the sensors, or electrical tests that can be performed? Wondering if I can do an "inspection" of the sensors to see if replacement is necessary. Thoughts?


#9 99obw


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Posted 25 May 2004 - 05:52 AM

As far as warming the exhaust up, I recommend driving it around the block a couple of times, then leave it running and get your 22mm wrench ready. Rev it to 2500 RPMs or so for a minute or two, then immediately loosen the sensor. It needs to be just about as hot as you can get it. Be careful.

#10 Nug


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Posted 25 May 2004 - 06:46 AM

It's fairly normal for O2 sensors to basically weld themselves to the bung. Often, the threads are smeared off of the O2 sensor by the time it comes out. I generally just crank the new one in there, disregarding the condition of the threads.

Sometimes the O2 sensor breaks. There is often nothing simple about it.

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