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Catalytic converter code

I had my catalytic converter stolen three years ago. When I went to a muffler shop to get a new one, they said I would need to get it from Toyota and it would cost $1400. Choking, I said I would need to find something cheaper. I went to ebay, found one for about $350 and a friend put it on. Good, now I’m done. But six months after putting on the new catalytic converter, the check engine light came on. I could use a code reader to clear it and it would stay off for a little while so the first year I was able to pass emissions test. Last year the light didn’t stay off long enough to pass emissions, so I knew I would need to do something. I needed verification that the code (which everyone kept saying was the catalytic converter) was really the catalytic converter. So I took it to a shop, paid $90 to have it read in detail. They told me that my 1998 Toyota Sienna had three catalytic converters and that two of them were in the front of the engine and would cost $1000 each to fix. Choking again, I went to ebay bought another catalytic converter (this time not the whole assembly) for $60 and a friend had it put on for me at a welding shop. I past emissions again, but within two weeks, the check engine light was on again. I am now due to get emissions inspected again and the light will not stay off long enough to make me feel comfortable. So, I have gone to a Toyota dealership and they tell me that there is only one catalytic converter and it is the one I have replaced twice now (so much for my $90). Also, Toyota is now selling the catalytic converter for $1690. Spending this much on a thirteen year old car is not worth it. Any ideas out there about what is wrong? Oh, by the way, Toyota said that the cheap converters do not have enough platinum in them that is why the light is coming on. Any ideas? How can they make a OEM part that doesn’t meet specs? Do I buy another and hope it lasts longer than the other two? Anyone have a similar problem? I need help.

Can you post the exact DTC code or codes that your vehicle is displaying?
In most states, autoparts stores, like Autozone, will read the codes for free.

Is this a V6? It may have two pre-converters up near the exhaust manifolds.
How many miles on the vehicle?

The 98 Toyota Sienna only came with a V6, and it does have two pre-converters located up near the exhaust manifolds. It also has a 3rd converter after the Y-pipe joins.

The code I am getting is P0420. I took it to a mechanic last year who said that it did have two converters up front, but they would cost $1000 to fix so it would be better to just replace the cat every year. I went to my local Toyota dealer this year and they said it only has one cat and that is underneath. There is a little over 180,000 miles on the car.

The P0420 code seems to be a problem with Toyota’s. What it is looking at is the output signals between the front and rear oxygen sensors. It is measuring the efficiency of the cat that is between the O2 sensors.

The number one thing to check is for leaks in the exhaust system. Any leaks will fool the sensors. Next is the sensors themselves, the rear sensor usually being the problem. Water or moisture in the connector of the rear sensor can cause this. What is usually not the problem (but it could be) is the cat itself.

On my 1999 Camry 4 cyl. replacing both 02 sensors (actually the front one is an A/F sensor on Cali. models) fixed my P0420 code problem. And cheaper than replacing both cats.

I’m unsure how this particular vehicle is set up, but Toyota these past years has taken to building a primary cat converter into each exhaust manifold and having a secondary comverter well down the pipe. The only converters monitored are the primary ones.

As Keith noted, look for a leak in the exhaust system. This Toyota has a 5-inch length of flex pipe built into the Y-pipe. That flex section often begins to leak - especially at this age.

If it were me, assuming P0420 is the only code you’re getting, I would look for:
1: A leak in the exhaust
2: A faulty rear O2 sensor
3: Lastly, suspect the rear converter.

The Y-pipe, which includes the rear converter is available new for about $500 from Bosal (sold via Autozone and others).