Plug it in, clear the code, and be done with it.

I get the P0420 code often enough on my 2000 4Runner, and yet for the 5 years now that I've been clearing the code, the thing seems to still work fine. Only 155k miles though.

Autozones I've visited recently. They used to clear but I think they've changed their minds about clearing codes (liability?). If you really wanted to get a code cleared you could still ask.

The last time I had an issue, they refused to clear the code and told me it was against company policy.

I believe that disconnecting the battery for a few minutes will still clear codes on most older cars, however.

For people like me who drive cars into the ground, a code reader/clearer is almost a necessity. I wouldn't go to Autozone to clear a code all the time.

It's annoying because the check engine light is almost always on, and how would we know if something else starts going on.

But he said it's expensive to fix and doesn't really affect things, so leave it alone.

It cost about $900. It doesn't matter in your case but I realized too late that it would have been covered under warranty but the non dealer shop I brought it to never mentioned that. I held off until I needed an emissions test and at that point I was over the miles for the catalytic converter which was less than the drivetrain warranty . I will definitely keep track of what is covered in the future and at what miles.

If not, just ignore it.

It lived a good life. Park it on a train track, report it stolen in a few days. Not sure what fair is on the repair offf the top of my head but it depends on which cat it is and how much of a pain in the ass it is to reach it, and on what else may need to come out to actually pull the thing off the car. There are times when I think Toyota hires their engineers from a pool of guys who got fired by Audi for needlessly making things difficult.

Also, there’s a decent chance the code will go away after 3 trips. Might come back in a week, might come back in a year.

Obviously your results may be different, but when I was at ND my car started getting half the gas mileage it used it. Went from ~400 miles a tank to ~250. Also was accelerating a lot slower. I took it to AutoZone and they ran the codes for free, and that's what it was. I remember because it was a pain in the ass to find a mechanic who would work on it in South Bend (It was a 10 year old Audi - not my smartest purchase, but still the most fun car I've owned. I don't own any, now).

But yeah, it was the O2 sensor that had gone bad. According to the guy who fixed it, that resulted in an incorrect mix of fuel and oxygen in the engine, or something like that, which caused the gas mileage to plummet. I know nothing about cars. Ended up costing me a couple hundred bucks.

And install Torque app on cell phone. Pair devices, scan for codes, clear codes. Cost around $20-25.
It's an emissions problem, won't really effect how the car runs. Whenever check engine light comes back on, scan and clear again.

2011 Cadillac SRX: $1900

2010 Mazda 3: ~$1200

So I would definitely guess it's in the $1k range. As far as I'm aware, the issue isn't the part cost. They require quite a bit of labor to replace, which drives up the price.

No clue on the O2 sensor issue. But I know that in both cases for us there was no doubt that it was the catalytic converter causing the issue once the mechanic (whom we trust, and has always been fair) looked at it.

It probably will but sometimes old cars throw codes.

You better have a rainy day jar in your kitchen as this will cost you some serious coin

That code is nearly always a cat. You have two on that car. For one side I would expect to pay $750 for the work, $1,500 for both. Your code is just for one side, but the other side will likely need replacement in the future. You can do them yourself but it is not a fun project.