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Catalytic converter dead again?

I have a 2003 Toyota Matrix with 145,000 miles. My check engine light just came on and when I pulled the code it was P0420 - Catalyst System Efficiency below threshold - bank 1.

I got this code at this time last year and the toyota dealer said that I needed to replace converter for $1400!!. I did that ( with a non-toyota part - much cheaper ) and one year later the same code appears.

I am not seeing a drop in fuel efficiency.

Is it likely that the cat has failed again, or could there be some other issue? Can I clear the code and pass state inspection? The state requires a code scan.

One possibility is that there was some underlying uncorrected problem last year that killed the converter and that problem has now killed the current converter.

Those Little Guys (Catalytic Converters) Get Blamed For Everything !

I suppose it could be a cat, but odds are that it’s more likey something else setting that PO420, one of the most common DTCs that turn on a CEL.

Link to obd codes site - PO420:


MANY MANY times it’s NOT the converter…but instead one of the O2 sensors. The code is generated by the O2 sensor reading. Well if it’s BAD it’ll deliver a FALSE reading to the computer saying the cat is bad.

ANY DECENT mechanic is first going to TEST the O2 sensors to see if it’s them or the cat. Any mechanic that sees this reading and then says immediately says it’s the cat without first testing the O2 sensors is a CROOK.

Catalytic converters are killed by 3 main causes:

Overly Rich mixture - ecu receives faulty information from the MAP/MAF sensor, or the O2 sensors and overly enrichens the mixture - or a fuel injector is stuck in a spray mode

Overly lean mixture - ecu receives faulty information from the MAP/MAF sensor, or the O2 sensors and overly leans out the mixture -

Engine oil contamination - Bad oil control by the engine, typically the piston rings, or the valve guide seals.

Your engine is known for failed oil control rings.
How much oil does your engine use every 1000 miles?


Unfortunately I don’t know how much oil is used per 1000 miles.

Sounds like I need to have the O2 sensors checked. If those are OK, then the CAT is the issue right. Of course that does not tell me why the CAT failed.

If I clear the code prior to inspection, will it pass?

Side note:

A friend of mine suggested clearing the code and filing the tank with mid grade gas prior to inspection. He said that it would pass the OBD scan.

Any thoughts?

“Sounds like I need to have the O2 sensors checked. If those are OK, then the CAT is the issue right. Of course that does not tell me why the CAT failed.”

Not necessarily. I have seen other systems (fuel/air delivery, EGR, etcetera) set that code and not set a code that would help one identify the real problem. Sometimes the problems are intermittent in nature.

Many people with code PO420 only, replace the cat, only to find the code comes back in short order.


If I clear the code prior to inspection, will it pass?

Not sure what’s done in your state…

But here in NH…if the code was cleared in the past x-days (not sure what x is) then it won’t pass inspections.

Type of gas will make NO difference what-so-ever.

As I said previously…check with the state…but in many states like NH you just can’t clear the code. The car has to have so many days since the last time the code was cleared in order to pass inspection. I’d be surprised if that’s not the case in ALL states.

This code seems to plague Toyota’s far more than any other manufacturer and Corolla/matrix models more than other Toyota’s. My daughter has an 03 Corolla that has had this code for over a year now. The code is saying that the rear O2 sensor is putting out a signal identical to the front sensor which is interpreted as the cat isn’t having any affect.

I have monitored the outputs on her O2 sensors and they were both acting as they should and not following each other. I have a friend that had a new 09 Corolla that had teh same code and the dealer was unable to get it to go away by replacing parts. Eventually they got it to clear up but would not say what they did and did not provide the usual work order. They would not provide any paperwork on that visit.

I am starting to suspect that there is a CPU problem and Toyota is using their usual tactic of hiding all problems.

Keith, So After A Year, That Cat Was Still Good ? There Ya Go. That Problem Makes Things Tough For People In These Inspection / Registration States.

I too have learned to turn off CELs, check and monitor carefully before jumping to anything drastic.


“Unfortunately I don’t know how much oil is used per 1000 miles.”

When you check your oil, do you sometimes need to add oil, and if so–how much do you add, and how often?

Ummm…you DO check your dipstick periodically, don’t you?

It sounds simular to a problem Im having with my 2000 miata, I had a new Cat put on it a week ago, and the cat fell apart (the mechanic said) said that the honeycomb part inside the cat was distroyed to pieces. Hes still trying to figure out whats causing it, but refuses to put another on the car until he figures why…

Keith, it seems that Toyotas made starting at about 2004 expend their cat comverters at about 150,000 miles. I had to change mine just a few months ago. I’ve noticed this just recently as these vehicles reach that mileage.

Toyota has taken to building the primary cat converter right into the exhaust manifold. The goal is apparently to get it heated up quickly and in all ambient temperatures, the purpose being emissions reduction. Putting it just past the exhaust ports accomplishes that, but I’m starting to wonder if that’s also affecting the lifespan of the catalyst. Perhaps it’s getting contaminated more just past the ports, perhaps the hotter temperature is causing something to cook onto the surface that used to pass through before, I really don’t know.

Are the rest of you seeing this phenominon in other makes as well? My enquiring mind wants to know.

Ladyntejas, a crumbling ceramic substrate suggests extreme heat and/or shock. Extreme heat usually comes from lean operation…analogous to using a bellows in the fireplace, the extra oxygen makes the fire hotter. This should, however, post a fault code.

Another possibility is heat buildup, like if the exhaust past the cat converter is restricted. This can be tough to diagnose because the engine will run fine at idle, but when it’s running restricted at 3000 rpm the heat builds.

Have you experienced any other codes? If so, post them.
Does your car have a resonator in the exhaust system? Perhaps that’s plugged.

I’m not generally a fan of throwing parts at a problem, but a partially restricted resonator or muffler can be tough to find. And it won’t post a failure code. In your case, since you’ve had three converters collapse and apparently diagnosis has been unsuccessful, you may want to suggest to the mechanic that he replace the rest of the sxhaust system.

Your problem is not the same as Federal102’s, but I’m glad you joined the conversation. Hopefully we can help you both.

I have to point out that the kind/brand of aftermarket cat you put in may have made a difference. If you look carefully at the warranties, you will see that many of them are only warrantied for a year, and some are only warrantied for physical integrity, not their catalytic function.

Well, I had Toyota check the O2 sensors etc and they told me that everything was OK, but the CAT was not performing to Toyota’s “exacting” standards. They recommended that I replace with a Toyota converter at $1500. I declined and replaced the converter with a different non OEM part with a 5 year 50000 mile warranty. The car passed inspection, but now 4 months later the dreaded CEL is back. At least I can ignore it until inspection rolls around in 8 months, when I will replace under warranty and start the cycle again!!

In dc if u show receipts for $850 you can get the inspection waived

Did that statement apply 5 years ago ?

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