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Catalytic converter conundrum

I have a 2002 toyota camry with 183,000 miles. Just recently, when the weather really dips down into the 20’s for more than a day or two, the check engine light would come one. My mechanic says it’s my front catalytic converter.

If there are two converters in there, and the emissions test cares about what comes out of the end of the tail pipe, why is the sensor in the middle? Is the second converter a half powered model, or is it designed to only remove a certain extra component from the exhaust? Most importantly, if the sensor were moved to the end of the exhaust system, would that fix my problem?

I don’t know the specifics of your system, but I do know that oxygen sensors are what tells the computer when the catalytic converter(s) are operating below normal efficiency, and it’s often the sensor the goes bad, not the cat.

Since the cats are a lot more expensive than the sensors, I’d have the sensors tested first (they can be removed and tested) and replaced if necessary.

There should be two oxygen sensors, one above and one below the first catalyst. Usually the top sensor fails first, but I replace them in pairs.

If yours are the original sensors with almost 200k miles on them, I would not spend money to pay someone to tell me that they are getting old (which they are by now), and then pay him over $100 each for new sensors. I would buy a new ones on line and put them in myself. It will cost you about $80 each for sensors that fit or $40 each for generic fit that you have to attach your old plug to.

You have literally nothing to loose by trying new oxygen sensors, because if you do need a new catalyst, you will replace the sensors as well.

I am not certain, but I presume that the two catalysts are completely different. One promotes reduction (removal of oxygen) of NOx, and the other promotes oxydation (addition of oxygen) to CO and VOC.

You can’t relocate the downstream the oxygen sensor, and even if you could, doing so would cost more and be more work than replacing the sensor.

People often scold me for “throwing parts at the problem” but if I have to remove a part to examine or test it, and it is a part that is subject to failure and it has over 100k miles on it, there is no point in testing it because I am not putting that old part back on my car.

I have a 2005 Scion tC, which, if you have he four cylinder, has the same engine and exhaust system you do. There are two catalytic converters, one built into the exhaust manifold, the other mid-pipe. Only the primary converter, the front one, is monitored. And when the converter becomes marginal, it will exhibit exactly your symptoms; a CEL when the cold weather hits disappearing when it warms up. You have typical mileage for this system to begin these symptoms, and it’ll need a converter. I changed mine out at about the same mileage.

NOTE: if you go to an independent shop and request a direct-fit aftermarket converter, you’ll save a ton of money. “Genuine Toyota parts” will cost you a bundle.

Sincere best.


I will assume you have the 2AZ-FE 4 cylinder . . .

Check this out

BTW . . . if you do get a cat installed . . . factory or aftermarket . . . do NOT forget to get that software update, or that P0420 might come back

Food for thought . . . @Manolito is correct about the sensors. In some cases, the degraded sensors are causing that P0420 code. If you’re going that route, I would recommend getting genuine Toyota parts. If you want to save a few bucks, get Denso sensors.

Mechanics don’t remove oxygen sensors to test them. I’m sorry, but I’ve been doing this awhile, and I’ve never seen anybody do this. At 200K, there’s no point. Use your scope, DMM or scan tool to diagnose the oxygen sensors. Or replace them. But don’t remove them to test them.

As a matter of fact, I recommend you get the software update no matter what you do. In some cases, the original software was a tad too sensitive.

Cats can get ruined by

Blown head gaskets
Excessive oil consumption

If any of this sounds familiar, a new cat might not live long