Catalytic Converter Damaged By Shop — But Customer Pays?

Does a consumer have a right to influence whether a catalytic converter on a car over 10 years of age is repaired vs. replaced? The car is a 2004 Subaru Outback but the catalytic converter in question is under five years (replaced twice by Subaru under warranty, most recently in 2016, along with an ECM reprogramming that was supposedly going to prevent the replacement from going bad).

It is unclear if a P0420 — a check engine light which can apparently even be triggered by low quality gasoline, among many other reasons — qualifies as proof that yet another catalytic converter replacement is required on this car. (Car passed a smog check less than a year ago because the P0420 is only intermittent. It isn’t due for another smog for nearly a year.)

When asked, the shop in question claims they are unable to confirm if the catalytic converter was failing because they are unable to get the 02 sensor back on due to stripped threads. The car had head gasket work in 2019 and the concern was the P0420 code may reflect cat damage because the CEL briefly appeared about a month after the HGs were replaced and has been going on and off ever since.) When asked why they didn’t test the catalytic converter for internal failure before they removed the 02 sensor — which they claim to have found cross-threaded — they offered no real explanation.

If the cat has failed internally it is a moot point that the shop also stripped the threads while attempting to remove the 02 sensor — but they haven’t proven anything other than they damaged our cat trying to address exhaust leaks — which they suspected as the cause of the P0420 code. It seems as if we are being asked to pay for an OEM cat not because the shop adequately determined that the old one required replacement but because they damaged it. (The shop says they warned us that “it may be necessary to replace the cat” but we took that to mean if the cat had failed internally — not merely because they damaged it prior to making a diagnosis.)

The shop is pressuring us to pick up the car in a non-drivable state after essentially holding the car hostage to a demand for an OEM cat replacement. We think we should be offered a lower cost option — at least one that would work long enough to re-mount the 02 sensor and properly test the CAT for internal failure. However, the shop is using their California STAR-certified smog test station license as reason to deny any/all workaround as an illegal "modification”.

I found at least one article online claiming that in California, a car over five years of age without any remaining emissions warranty is supposed to be exempted from an OEM cat requirement, but it’s neither here nor there if the shop that has our car in pieces refuses to install an aftermarket cat. (I’ve posted this question elsewhere online and everyone is siding with the shop, saying we have no recourse simply by virtue of the fact that the cross-threading was mentioned before they stripped out the threads — but we had no specific discussion what would happen if the threads stripped and assumed the reference was to the fact that a back pressure or vacuum test may indicate that the cat itself had gone bad.)

Can a shop legally push a customer’s back against a wall for the highest cost repair option when there may be other things they can try first — like using a $6 oxygen sensor thread chaser to restore the grip for the 02 sensor?

The car has about 175K on the odometer but we had hoped to make it to the 200K mark. Problem is, for three weeks we have been at an impasse with the shop — which may force us to scrap the car! So are we being told the truth when they say their STAR smog-check station status ties their hands? Does it also mean that if we take it to an exhaust/muffler shop for a bung or tap, it will mean an automatic fail the next time the car is presented to a STAR smog shop? (They claim it’s not worth taking it elsewhere because a repair of the existing cat will cause the car to fail future Star-certified smog checks if such a “modification” is discovered.) Are they telling us the truth or just trying to extract $3,500 from us for the catalytic replacement? Is a customer really liable for the entire cost of replacement in this situation or is there any legal requirement that the shop that damages such an expensive component share in the replacement cost?

It’s entirely possible that it was crossthreaded the last time the cat was replaced. Trouble is, it’s your word against theirs.

That said, I’m not sure why they wouldn’t just chase the threads with a tap, at least so they could get the sensor installed long enough to see if the cat is working. I can’t imagine that even California’s strict emissions laws would outlaw fixing bolt threads, though I can’t say for certain since I don’t live there.

What does that have to do with it? Last I checked, aftermarket cats were legal in California provided they pass an evaluation from the state. However, because of that extra regulatory step it wouldn’t surprise me if the legal aftermarket cats were almost if not as expensive as OEM.

That aside, if you really think this shop is “holding the car hostage” and is trying to cheat you, why would you want them working on your car?

This is a car forum, not a lawyer forum. Anything we say on the law is layman’s guessing, not solid advice. And from my layman’s perspective, in general the shop is allowed to refuse to do any work they want to refuse to do. You can always go get the car and bring it to a different shop. Stuff like this generally falls under civil law, which means the shop can and will get away with anything it wants unless you sue them and win. And oftentimes it’s more trouble and cost to bring a lawsuit than it’s worth given the potential judgment you’d get, if you won.

Well that’s where we come back around to the first thing I said - Because the cat was replaced before, and therefore the O2 sensors were installed previously by a different shop, you cannot prove that this shop damaged the threads. But what you can do is not give this shop thousands of dollars for refusing to chase the damn thread, because a repaired thread “will cause the car to fail future Star-certified smog checks if such a “modification” is discovered” is a line of utter BS. A) like I said, I highly doubt that repairing a screw thread would violate the law and B) even if it did, no one is ever going to know the thread was repaired unless they go in there with a flashlight and a magnifying glass, and why would anyone do that? They don’t remove oxygen sensors at emissions tests, and you can’t even see the thread when the sensor is installed.

Were this my car, I’d yank it out of the shop tomorrow and bring it to a shop that isn’t quite so full of it, but I’d also realize that no matter how much I suspect these idiots wrecked the thread, I’d never be able to prove it, so that’s one I’d just have to let go.


I guess I don’t disagree to take it somewhere else and put a non OEM cat in it unless you are in California. Might want to check first with another shop though since you don’t want to do the work and have to do it again. Still there are cheaper replacements around. But normally you don’t cross thread something taking it out but rather putting it back in again.

Edit: $2-500 at Rockauto. Put it on yourself. But why does a car that old need a smog inspection?

“That aside, if you really think this shop is “holding the car hostage” and is trying to cheat you, why would you want them working on your car?”

I say that only because they’ve had the car ~3 weeks and have shot down all attempts to arrive at a compromise that will return the car to a drive-able state — even though they admit they stripped the threads while removing the O2 sensor.

On a much older topic, I describe the prior issues with repairs. After encountering poor quality work we pulled up stakes and went elsewhere — four now and counting. After taking the car to an independent shop for major work in 2019 that didn’t go well — a head gasket replacement that immediately had to be repeated after the belts dropped one day out of the shop, among other issues — we turned to the dealer for a P1091 code that the first shop blamed on a failing ECM. Unfortunately, the Subaru dealer in our area doesn’t have any veteran techs or management that is familiar with this model year so after six weeks of few updates on the car, I reached out to Subaru of America for help because our calls were being routed into voicemail (that the dealer didn’t return, falsely claiming they were closed due to COVID-19). That was our first “introduction” to the only Subaru dealer within ~80 miles.

The best SoA could recommend last year at this time was that we tow it to another dealer (all we wanted SoA to do was verify our service record since we couldn’t get a straight answer what was going on with the car.) Eventually we had to have the car towed from the dealer to another independent repair shop after learning that the dealer attempted to install — but broke — a “tumble generator valve” that was supposed to fix (but didn’t) a P1091 code. (After trying a second time without success, the dealer reinstalled the original part, then hit us up for over $1K worth of intake manifold cleaning that they refused to warranty as a solution for the check engine light). The shop we towed it to found that the alternator assembly was physically obstructing the operation of the valve due to improper installation by a mobile mechanic — which begs the question why the dealer didn’t catch the problem after doing the TGV swaps 3x over.

After only three years of spouse and I living where we do, we have yet to replace the trusted mechanic we had for over 15 years previous to our move. The shop that presently has our cat in pieces was originally asked to take a look at what appeared to be a failure of an accelerator peddle assembly they installed a few months previously (car lost power and threw codes for P0638, P2109). At the time of the peddle assembly repair, the shop had said there was an “exhaust leak” and speculated that if they fixed it, the intermittent P0420 code would disappear. Since we’ve had a history of the car requiring a cat replacement twice under the original emissions warranty — which may suggest there’s a problem elsewhere that may be killing the cats — we asked the current shop to evaluate the cat before we put money into the throttle body. At that point — late on a Friday — the shop owner mentioned that it may become necessary to replace the cat, but offered to “take a look at it on Monday”. Hearing that, we agreed to have the shop evaluate the cat, thinking it would involve less invasive things like checking the temperature, pressure and whatnot. Instead, the following Monday we learn they removed the 02 sensor and stripped out the threads. (They admit it, which is why it doesn’t make sense that they’re not willing to keep their customer happy.) Yes, she had mentioned the Friday before that the O2 sensor appeared to be cross-threaded — but threads that strip elsewhere on a car are repaired all the time. We had no way to predict that stripping out the threads may be A) inevitable, or B) would force the issue of replacing the cat.

Although I know this is a car repair forum not a legal forum, I am hopeful that someone with CA experience can weigh in on the question of whether a Star-certified shop would be under some obligation, as a means of keeping their license in good standing, to shoot down any and all workarounds, even if only temporary to determine whether or not the cat was failing internally. Perhaps that question is beyond the scope of the forums — although one can always hope. Still, as a Star-certified shop, it’s surprising to learn that they didn’t first check the cat pressure, temperature or even comment on whether the cat had turned blue — all things they could have done first before proceeding to pry off the O2 sensor. All I know is that when I asked what testing was performed on the cat, she didn’t have an answer — blaming the fact that the O2 sensor had been removed for failing to make a determination.

The shop knows a bit about our history with the prior work so I guess they thought they could simply say that the cat “may need to be replaced” and that somehow constituted our consent to proceed to replacement — even though it was never shown that replacement would have been necessary otherwise. The level of bad luck spouse and I have had with this car since relocating — despite being original owners of a fully maintained and formerly rock-solid reliable car — has been nothing short of unbelievable. :astonished:

The car doesn’t need a smog inspection for almost another year. The P0420 code started coming on every now and then a few weeks after the head gaskets were repaired in 2019. There were a bunch of other problems with shops taking an inordinate amount of time to diagnose another check engine light (P1091), causing us not to have the car for nearly six months out of the past year. So despite the amount of time since the HG work in summer 2019, the car really hasn’t had a whole lot of more miles put on it.

When we were told recently after losing acceleration power and throwing new codes that the car requires a new throttle body — $1,500! — we asked if the new codes had any relation to the earlier P0420 code (for the cat). We decided to investigate the cause of the P0420 because in CA the smog checks on older cars continue on and on (pretty much every other year).

We can’t really prove who cross-threaded the O2 sensor. Most likely it was the shop that did the head gasket work, rack-and-pinion and drive shaft replacement — but officially they never touched the cat (unless taking it off was necessary for the other repairs but it’s doubtful they’ll admit it). The only other party that might have cross-threaded the O2 sensor was the dealer back when they replaced the cat in 2016 (I have a very vague recollection of something being said about an O2 sensor being “stuck” but it was never written down in the service history, we had no idea what the significance would be down the road — and the dealer has since changed ownership). It’s too late to prove who did what on the cross-threading — but if the current shop had a hard-and-fast rule that they won’t repair thread damage in this location they should have told us that before they pried the O2 sensor off. Instead they claim they’re in the clear because they warned us that the cat “may need to be replaced”. However, the context for that conversation was that it may have sustained damage from the overheating back when the head gaskets failed — not an “agreement” on our part to replace the cat for no other documented reason than they stripped the threads.

Are you trying to talk us into agreeing with you or are you looking for advice?

@shadowfax gave you very good advice. I suggest you take it.

Tow the car if you must but pull it out of that shop. I’d also suggest not buying a Subaru for your next car, but that’s just me.

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I would scrap a 17 y.o. Subaru in a heartbeat.

Dang, covid folks have time on hand to write 1300 word novels on car issues.

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Yep, California all bets are off. They don’t want old cars on the road from what I gather.

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Apparently Car sleuth does not know when to cut his losses . Of course that opinion comes with out reading all his life story.


I think it’s ridiculous the shop is saying that repairing . . . or even replacing the oxygen sensor bung . . . would constitute a failure because it’s a “modification”

First of all, it’s a repair, not a “modification”

And second of all, it seems the shop really is determined to sell the customer a new cat, fully well knowing it can be properly repaired for not much money


Two things here from the viewpoint of a guy who runs an auto shop and spent over 20 years as a state authorized emissions specialist:

  1. If the oxygen sensor threads strip as we are removing the sensor to do some testing, the damage and repair is on the customer. Even if we were the ones that replaced the catalyst and installed the sensor previously. We are not going to accept responsibility for every stripped thread or broken fastener out there. Things happen and stuff breaks. The customer pays for the repair because the customer owns the car. Simple as that.

  2. There’s no way on earth I would ever sign off on replacing a $1000 catalyst because the O2 hole is stripped. I would repair the threads (there is a device that repairs the threads from the inside out).

Now there are some catalyst designs where the downstream sensor screws in to the catalyst housing itself rather than into the pipe behind it. If this is the case then cutting and welding in a new bung would not be possible, nor can you relocate the sensor down to the pipe. But a skilled mechanic can surely repair the threads enough to install a sensor one time.