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CEL after catalytic converter replacement

Hi guys!! I had some good luck posting here last time I had an issue so here goes.

I’m trying to get the car to pass emissions and have a CEL. Have the dreaded p420 code.

We replaced the cat and the performance drastically improved so I’m fairly confident it was necessary. I’d been trying to complete the drive cycle so I can renew the car, and then the CEL came back on with the same code.

Prior to the replacement I had disconnected the battery to clean the corroded terminals and honestly forgot about the light. Went to take it to emissions and got the not ready indicator. I was waiting for the evap and cat. Drove on a high speed road and the light kicked on. I want to point out the o2 sensor was in a ready state but there had been no light (if it matters).

My friend had a similar issue and his turned out to be that they had a CA model car and needed a better cat (or something?) I checked my car and it looks like mine might be the same issue.

I took the car back today and asked them to check and also asked about the installed cat. I couldn’t find much online since the info on my receipt is a little generic. They insist it being a CA model doesn’t matter. I confronted the technician about it and he said it’s a very high load cat because subaru vehicles are choosy and insists that isn’t it. First he says the computer may have kicked it by mistake and I may need to drive more. Then he said I need to replace the o2 sensor next since the cat is new (but running kind of hot) despite the results from emissions prior. They want me to go back first thing in the morning for the additional repair.

I don’t know if they’re being honest with me. I honestly learn about my car as stuff breaks and so I’m very confused.

It’s a 2002 Subaru Outback if that matters and I attached my emissions label.

The catalytic converter must be C.A.R.B. compliant for it to work in the vehicle.


That’s what I brought up to him and he said that wasn’t true

Then find another mechanic!


I’ll have to call around and just tell the guy I can’t show tomorrow. But that was my concern — that they’re not being honest. I specifically asked if it was a CARB cat and he said no I didn’t want that, and it was high load enough.

I don’t know much about this stuff. They went and installed the wrong part, am I just… out that money??


Just tell them they violated EPA/C.A.R.B.regulations and can be fined for doing what they did if reported.

It’s called tampering/disabling emission controls.


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Does this apply if I’m not in California though?

It’s a federal regulation, and states follow the same regulations.

California more so.


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Here’s to hoping I can actually get my money back hah.

while tackling that P0420 code, you’ll also want to make sure you don’t have any exhaust leaks, which could affect the oxygen sensor operation

The code didn’t set by mistake

And you drove the car long enough

if he cleared the code and it came back, that means you drove the car long enough for the catalytic readiness monitor to run to completion. And it failed, therefore the exact same code came back

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Would the leak have been an obvious thing for them to notice?

And thank you for clarifying the fact the code is still here to haunt me for sure. I was gonna drive it a bunch before taking it to a mechanic for a second opinion.

This may be the first time that I have ever disagreed with Tester. The engine computer has no way of knowing whether the catalyst is California compliant.
I have seen authoritative-looking statements on the web that said that a California car would not run properly unless it has a California-compliant catalyst. That is not true. What is true is that a catalyst car is difficult to get to run properly with no catalyst at all. A misunderstanding of this fact is likely the source of the claim.
The only real difference between a California compliant catalyst and a non-California compliant is that California has the ridiculous requirement that aftermarket catalysts be guaranteed for 50k miles. Of course, many of them fail within that period because they are installed on cars with mechanical problems (that caused the original catalyst to fail). Therefore, the few aftermarket vendors willing to deal with this requirement double the price of their catalysts to cover warranty claims, and then stamp a CARB approval number on them.
The most common cause of a P0420 code is an oxygen sensor that is old and slow to respond. The computer cannot directly measure catalyst performance and must rely on the signals it is getting from the oxygen sensors. My experience is that most of the time, a “Catalyst System Efficiency Below Threshold” code is cured with fresh oxygen sensors, and the catalyst continues to work just fine for another 100,000+ miles.
…But be sure to check for leaks upstream of that new catalyst.



The downstream o2 was just replaced. They didn’t tell me there were leaks if visible signs, I did ask how everything looked. I’ll update if the CEL stays off

The downstream sensor and catalytic converter were the solution! It’s a CA model car but we have what I think is a mid-grade cat on it, and it passed fine! Thanks for the responses guys it was very confusing.

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I seem to remember the issue being that if the inspector doesn’t see the proper CARB certification placard on the cat, it is an automatic failure. I don’t know if the inspectors get that picky in the other states that CA emissions, though. I live in one of them but I have never had to replace a cat.

Other states they won’t care. I’ve read you’re supposed to keep California equipment in a California car, whatever that means. My extreme confusion was that my friend had a similar issue — replaced the cat and still had a light. A mechanic said the computer checked for CA level emissions and they had to put a different cat in. Well, they did and it passed afterwards which was largely why I wasn’t sure what to do. I believe they put a mid-grade one in mine and it worked so I don’t know if there’s truth to it or what was really wrong with my friend’s car.


and not just any CARB cat . . . the inspector is supposed to look up the number and verify that it’s actually approved for that exact vehicle

And when I say exact, I mean make, model, model year, engine type, size, and a few other factors

An acquaintance of mine failed a smog inspection because of the aftermarket cat

It was a CARB cat . . . but not approved for her exact vehicle