New Car, Bad Catalytic Converter


Last weekend I bought a 2002 Toyota Sienna from a small used car dealer. It has 107,000 miles. I didn’t have time to get it inspected and managed to negotiate a whopping 3-day warranty.

Two days in and my check engine light came on.

It came up as a bad catalytic converter – P0420. The dealer had me take it to a local mechanic, who told me that it wasn’t an O2 sensor, but that it was the catalytic converter. He said the converter wasn’t plugged. It wasn’t getting hot enough. He said the Sienna performed fine on an emissions test.

The dealer then sent me to a muffler repair shop that by all accounts is pretty reputable. He insisted that the converter should not be a problem at this point and “decarbonized” the engine and the converter. He finished last night. As of now, the check engine light has not come back on.

I’m now past my three days, of course, and am concerned I bought a lemon. I’m also worried about what may have caused the converter to foul up. Nobody has been able to tell me.


* Should decarbonizing work and thus keep me from buying a new converter?

* Even if it does work, is there ANY WAY I can tell what caused this problem – without paying through the nose?

The van runs perfect. It has shown no outward signs of problems and is smooth and not noisy. We love the van. But I don’t love the prospect of spending thousands to fix an uncertain problem.


The three day warranty didn’t raise any red flags?

First of all, P0420 does NOT indicate a “Bad Catalytic Converter.” It indicates a “converter operating below threshold,” which is quite different.

This is often caused by a bad O2 sensor or some other problem, but doesn’t mean the cat needs to be replaced.

Does the CEL come on when you turn to key to “ON?” It should, and then it should go off when you start the engine. If this is what’s happening you’re OK unless the light comes back on.

I’m not sure how one goes about “decarbonizing” a catalytic converter, or an engine for that matter, but if the CEL is operating properly I’d leave it alone.

Regardless, you should not need a new converter. Diagnosis of other components within the emissions system should be done to determine what’s preventing the converter from operating correctly.

Do you have any maintenance history on this van? There may be some work necessary to bring it up to date. Timing belt? Transmission fluid change? Please check the maintenance schedule to see what might be required.

That’s a good point regarding the red flag. But the warranty was to give me an opportunity to have the van inspected. I did – nothing major. Well, except for the check-engine light. The light didn’t go on for two days, and the mechanic would not have discovered a problem if I had it inspected first. So in a sense I’m glad for the warranty, because I got the inspection and the “decarbinizing” paid for.

The mechanic said he checked the O2 sensors, and said they’re working fine. But I do know that oxygen sensors on the 02 Siennas tend to go bad.

As for the regular maintenance, I have every intent on getting the schedule up to date, including the timing belt and a transmission fluid change. I’m also thinking of changing the plugs and, depending on how they look, the wires – just so I know when it got done.


A three day warranty is really generous. When I sell a car, the warranty is for 5 minutes or the end of my driveway–whichever comes first.

Actually, it sounds like the dealer is trying to do the right thing. I always expect to have to put some repairs into an older used car, but have almost always come out o.k.

I would keep driving it and if the check engine light comes on, go to a good repair shop that can diagnose problems. Don’t go looking for trouble.

You alerted the dealer of the problem within the 3 day limit. You should be covered. Do you have a record of the contact?

Yeah, I generally think the dealer is trying to do the right thing – in his defense. He didn’t really have to pay for any of this stuff. I do not intend to go back to the dealer at this stage. I just worry that I bought a lemon. A new converter is expensive and I’m not exactly free-flowing with extra cash after buying the Sienna. But it doesn’t sound like I need a new converter, only to figure out why it’s not operating up to par. Thanks for the help!

The dealer guided the inspection and the subsequent decarbonizing and paid for them both.