A bad secondary O2 sensor can also throw P0420.

And there should be only one catalytic converter.

 

There a 2 pre-catalytic converters, and 1 main converter. The pre-cats are smaller. One is built into the front exhaust manifold. The other is built into the y-pipe that merges both exhaust sides into a single pipe. There are various threads about the P0420 code.
See this very recent one for starters:
http://www.siennachat.com/forum/62-...roblems-maintenance-repair/20272-p0420-code-o2-sensor-replacement-question.html

A P0420 code can be caused by many things. Among them are exhaust leaks, a bad O2 sensor, or (truly)worn-out cats. There is a TSB(EG047-05) for the 2001-2003 model years. The engine ECU kicks off the P0420 code long before the cats are truly worn out. Toyota's solution is an updated ECU(with modified O2 checks) for $1500 or new cats(all 3) for about $1500. In the above mentioned link, one guy solved it by installing a simple diode inline with one of the O2 sensor wires. This essentially accomplishes the same thing as a new updated ECU at the signal end rather than in the ECU's code.

 

There a 2 pre-catalytic converters, and 1 main converter.

Thanks for the clarification. Made me look up the function of the pre-cats, and also wonder whether replacing the pre-cats on my 98 Sienna would eliminate the rich exhaust smell when the engine is cold.

 

One more thing... you asked "which cat?"

The following is hearsay via a Toyota Master Tech that I talked to a few years ago...
He said that the pre-cat in the y-pipe(Bank 1) is the one that matters to ECU in the cat test.
So if this is correct, the front pre-cat doesn't matter.... only the Bank 1 and final cats matter.

The ECU compares the waveform of the Bank 1 Air/Fuel(sometimes called an O2) sensor with the O2 sensor after the final main cat.
The waveforms should be different. The final waveform should have smaller amplitudes if the cats are performing correctly. When the cats are worn, the waveforms become too similar. The OEM ECU fires off the P0420 long before the cats are really worn-out.
Toyota's solution is for you to pay them a lot of money to fix their programming error in the ECU.(or buy expensive new cats)

 

you have a link for the diode solution? Thanks

 

Heard the 2001 is very sensitive to non-OEM catalytic Convertors i.e. Rosa / Walker is this true? has anyone with a 2001 Sienna had success using a non-OEM Replacement Convertor ?

 

I tried Walker aftermarket cats when I first got a P0420 code. I bought the y-pipe with the Bank 1 pre-cat, and a main rear cat. This corrected the P0420 code for about 6 months and the code returned. To finally fix my P0420, I updated the ECU with an updated used ECU that I bought for $120. These Walker cats worked with the new ECU for a while. But the code still returned again even with the updated ECU. I'm not sure if they were worn-out, or if I had an exhaust leak at the manifold flanges. When I replaced the Walker cats with my original cats, the P0420 was eliminated for good. The P0420 code may have returned with the Walker cats(and upgraded ECU) because of the exhaust leak. The Walker cats might still be good...who knows. I re-used the exhaust gaskets when I originally installed the Walker cats. This was a mistake. I could see where there was "blow-by" on the gasket. These flange gaskets are "crush" gaskets and they may not seal well a second time. When I re-installed my OEM cats, I used new gaskets.

Aftermarket cats are cheap for a reason. They have much less of the expensive platinum or palladium material. You get what you pay for. However paying for new OEM cats on a 16 year-old cars is not cost effective. Neither is paying for a new OEM ECU from Toyota.
Used upgraded ECUs are somewhat difficult to find. Unless you get an ECU from a Sienna that did not have an immobilizer, you will have to reprogram a tiny chip(93C56) on the motherboard to clear out the transponder codes of the previous Sienna's keys. However, if you can get the original transponder keys, a locksmith can adapt the keys to your lock. There are a lot of variables.

 

I think this thread has the diode info:

http://www.siennachat.com/forum/62-problems-maintenance-repair/30673-revenge-p0420-code.html

Geoffzie said in the thread:
"Rather than replace the CATS, I soldered a standard Diode from radio shack (P/N: 276-1141) into the sensor lead (blue wire on a Denso O2 Sensor) of each downstream Oxygen sensor (silver diode marking must be towards 02 connector/ECU)."

Since our Sienna's only have one downstream O2 sensor, you would only need one diode. If I get a P0420 code again, I will be installing a diode.
I just looked on Ebay, there is an 89661-08062 ECU(upgraded for a 2001) for sale for about $250. I think the diode would be a much simpler and cheaper option... assuming it works.

 

To clarify, you changed the front catalytic converter? Did you also change the lower oxygen sensor?

 

I changed the final O2 sensor. That was the first thing I tried... since it was the cheapest. I also changed both Air/Fuel sensors(some call them O2 sensors) on the exhaust manifolds. The A/F sensors were not all changed at the same time. I changed the front(Bank 2) a few years earlier due to a code. I changed the Bank 1 sensor while I was still getting the P0420 code.

I changed the final cat and the pre-cat that is built into the y-pipe with Walker cats within a few months of each other. For about 6 months, this seemed to fix it. Then the code came back. Either the Walker cats "wore out" enough to trigger the code, or an exhaust leak at the exhaust flanges finally triggered the code again. I finally replaced my ECU with an updated ECU that has the less stringent cat test and re-installed the OEM cats with new gaskets. Since then, no more P0420.

So my conclusion is that the OEM cats are good for a very long time with the updated ECU. I have 155K miles now on them. Without the updated ECU, you will probably get the P0420 code somewhere around 100K miles.(+/-20K miles) I have little confidence in aftermarket cats to last very long even with an updated ECU. Who knows how long the diode fix will work? Potentially, forever...depending upon how much it attenuates the signal of the final O2 sensor.

I have never changed the front manifold which contains the other pre-cat. From what I have been told(and read elsewhere), this cat (and the associated A/F sensor) is not involved in the ECU's cat test.

 

What would happen if you unplugged the final O2 sensor but left it in place, then bought a new final O2 sensor (a cheap $20 one that had the correct plug) and plugged the new one into the harness and then mounted it up under the car somewhere out of the way? When they say heated, does it get red hot? I ended up with a free aftermarket one and have wondered if I could do that, I still get a P0420 every month or so, been driving like for two years, drives fine.

 

If you connected the O2 plug to a sensor that is mounted outside the exhaust, then it would have no exhaust signal "waveform". It would probably recognize the invalid signal.

 

A few things before you start replacing parts:

How many miles are on your van? Have you ever had any other codes, such as for a bad A/F sensor?

My experience with my 2002 van was to have two bad A/F sensors, replaced, and then the P0420. It was solved by replacing the rear (sensor 2) oxygen sensor. All this was over 5 years ago, at about 150,000 miles, and I have had no codes since, and still have all OEM cats. I suggest replacing the sensors first. Like Robert says, the converters have a long life span.