Sorry about your van. People suck.

Start just replacing the oxygen sensor, which is responsible for the P0420. Beware, its mounting studs are likely rusty. See if that solves the problem.

It's likely the 2 A/F sensors are degraded too, though, so don't expect everything will be fine. One of the A/F sensors is very difficult to reach and remove, being located near the firewall; the other is a cinch, located next to the ATF dipstick.

Whatever you do, don't yet replace any catalyst. At least in my van, and in many others', it has been aged sensors that throw these emissions codes. If your catalysts are indeed bad, they'll be sticking in new sensors anyway.

Also, the ECU logic for emissions is too stringent, and Toyota has a new ECU, with better logic, though this is a $1200 part, too. A dealer should know this, and not just proclaim the converters dead. Finally, at least in 2002, all Siennas were California-compliant, and I think that's true also for 2001s; there are 3 catalysts.

Hopefully you have your own code reader, so you can accomplish these repairs and code clearings, yourself. Will you be doing the work? If so, get the repair manual under "General Discussion." This can also help you if you don't do the work, showing you where parts are located.

 

Your cats might be perfectly good. The 2001-2003 Sienna's have "faulty" ECUs(engine computer) that throw the P0420 code while the cats are still performing adequately. This happened on my 2001 Sienna. The original ECU catalyst test is too stringent. Toyota updated the ECUs to correct the catalyst test. However, they want about $1500 for the ECU. You can find it for less on the internet however(around $1000-1100). To fix the problem you have a few options:
1) Buy new OEM Toyota cats. They will pass the old ECU's catalyst test. Aftermarket cats probably won't pass for long...if at all. They are much cheaper because they have lesser amounts of the expensive catalyst(platinum, etc.)
2) Update the ECU. If this still doesn't pass the catalyst test you can now buy the cheaper aftermarket cats.(i.e. Bosal from Rockauto.com) The cheaper ones will now pass the less stringent catalyst test. If necessary, I would replace the y-pipe pre-cat first. Then if it still doesn't pass, I would replace the rear-main cat next. More on cats below...

A 2001 Sienna has 3 cats. There are 2 pre-cats and one final "main" cat. One pre-cat is built into the front exhaust manifold. The 2nd pre-cat is built into the y-pipe. The final "main" cat is built into a 3-foot straight pipe that comes after the y-pipe. From what I've read (and been told from a Toyota Master tech), the pre-cat in the y-pipe and the main rear cat are used in the catalyst test by the ECU. It compares the wave forms from the pre-cat's O2(actually Air/Fuel) sensor and the main cat's O2 sensor. The front pre-cat built into the manifold doesn't matter for the catalyst test.

There is a TSB on this ECU problem. It is EG047-05. The original ECU is either part # 89661-08060 or 89661-08061. The updated part is 89661-08062 or 89661-08063. Toyota only sells 89661-08063 now. It includes another update for knocking on some Siennas.

You can buy the ECU on the internet and install it yourself. But if your Sienna has an immobilizer, you will have to reprogram the keys when you first start the car after installing it. There is a procedure for this. It isn't difficult. I didn't have to do this as my Sienna did not have it. If your keys are all metal with no plastic case(containing a transponder) then you do not have the immobilizer.
You can buy a used 89661-08062 or 89661-08063 ECU... if you are lucky enough to find one. This is what I did. However, you will either need to find one from a Sienna that had no immobilizer, or you will have to have a chip in the ECU reprogrammed to a "virgin" state so that it will accept new transponder keys.(from your existing keys) An ECU from a Sienna with no immobilizer should already be in a "virgin" state. I had to reprogram the 93C56 chip on the ECU to a "virgin" state to get my used ECU to work. If you can do this, it is the cheapest way to go...but the road was long and hard for me to figure this all out!
One more note on the ECUs... The 2001-2003 ECUs cannot be simply "re-flashed" like many other cars. They are not downloadable. This is why you have to buy an updated one. The 93C56 chip is a tiny little chip on the ECU's motherboard that holds the transponder key values(256 bytes). It can be reprogrammed or replaced if you are good at soldering tiny things.

All that said... a P0420 can sometimes be caused by a simple exhaust leak. This throws off the catalyst test. If your exhaust has been removed and replaced for any reason for other repairs, the gaskets should have been replaced when this was done. The gaskets to the manifolds on the y-pipe especially should not be re-used. They are cheap "crush" gaskets. They conform to the original gap and may(probably will) leak if used again. Been there...done that...

If you click on my user name and go see all my posts, you can find other threads that talk about the infamous P0420 code.

 

I didn't mention the O2 sensors...
It could be caused by the pre-cat sensor that monitors the pre-cat on the y-pipe.(The tough one to get to...in front of the firewall) It is used in the catalyst test. Since the ECU compares the values from this pre-cat O2(A/F) sensor and the rear O2 sensor, it would come into play. However, the rear cat O2 sensor will typically yield a lesser signal(which is actually good!) as it ages. The ECU wants to see a difference in the two waveforms to see that the cat is actually working. A fresh new O2 sensor at the rear might only send out a stronger signal than the old tired one.

When I first got a P0420 code, I replaced the sensor and rear cat with an aftermarket cat. The P0420 came back.
Then I replaced the y-pipe and pre-cat with an aftermarket cat. The P0420 came back again.
I replaced the y-pipe's O2(A/F) sensor. Still had the P0420...
After I replaced my ECU, then my aftermarket cats passed fine.

One more note on the rear O2 sensor... It doesn't screw in like the 2 front A/F sensors. It has a plate with 2 bolts that hold it in. In rust belt areas, this has been known to rust apart and leak.

As pkjsr02 said, having a code reader will help you debug and monitor your results. When your cat finally passes the catalyst test and is "ready", you will know that the problem is finally solved. It takes a few "drive cycles" for this to occur.

 

Thanks for your help guys. I'm going to replace all the sensors and possibly try cleaning the CAT if it's not welded in or maybe use some sea foam. Just wanted to verify that i'm buying the right parts.

I need two Denso A/F sensors (234 9021) and 1 downstream O2 sensor (234 - 4163) ? I figure I'd rather just try to replace all of them at the same time. Will this cover all the sensors? I've been a little confused about how many O2 sensors there are aside from the A/F sensors

 

I had the same issue with the code. I first replaced 2 cats that were bad along with 2 front A/F sensors were replaced. A month later the light came back! I replaced the rear o2 sensor with a Denso sensor through amazon and the light hasn't come on in 4 months, so I believe its fixed. My biggest advice is to use either oem or Denso aftermarket sensors because other brand might not work with the picky computer.

 

You have it right. There are 3 sensors in the exhaust system. 2 A/F sensors, (often called O2 sensors) and 1 O2 sensor. As mjpendrick said, I would stay with using the Denso sensors. I used a Bosch on rear O2 sensor. It works fine, but if I were to do it again, I would get the Denso.

 

I just bought a 2002 Sienna CE a few months back. A week after driving it off the lot I got a check engine light. I went to Advanced Auto and they ran a check for me and came up with a P0420 code. I read up on the code and found many different things can cause this code. The common belief is that the code means you need a new catalytic converter but that isn't true. The code, or rather what causes the code to appear in the first place, can lead to your cats getting damaged but doesn't immediately mean your cat/s are bad (it's a generic code). Vacuum leaks, exhaust leaks, the engine running too rich or too lean, bad or old O2 sensors, and yes, a bad catalytic converter can all cause the P0420 code.

I went to my mechanic (a friend who owns his own shop) and he scanned it for codes; P0420. He immediately said it needed a new catalytic converter (which surprised me since I figured he would explain what types of problems can lead to the P0420 code, or maybe he'd dealt with this issue so many times in the past that usually it ended in replacing the main cat?). Anyway, I told him I couldn't afford $1500.00 for a new catalytic converter and asked him what I should do to pass the emissions test.

First he cleared the code and then told me to fill the tank with premium gas (another surprise since all my research has showed there isn't much of difference between premium and regular gas besides the octane rating). He told me drive at least 150 miles on the new tank of gas (both city and highway) and then take it in to get tested. He explained the drive cycle to me and how the internal monitors needed to be reset or the Sienna wouldn't pass the test. Anyway I drove about 120 city miles and about 70 highway miles (The last of the highway mileage was put on riding to the testing station). I drove into the testing station with exactly 190 miles on the tripometer. Surprisingly, I passed the emissions test!

Unfortunately the damn check engine light came back on 30 miles later!

I did do two fuel treatments in the month before the test. One of them was a Lucas product but I forget the exact name of the treatment. The other brand I forgot. If anyone needs to pass an emissions test they can replicate my experiences above and maybe they'll have the same luck.

As for getting rid of the check engine light...It seems it can go from very simple to fix to very difficult (or expensive!). Apparently the ECM is overly sensitive on Sienna's between 2000-2003 and there is no fix that I know of except to by a newer ECM and I'm not going to do that, too much money. Another way to fix the problem (getting rid of the CEL) is to get an O2 sensor extension (I might go that route). I could just use some electrical tape to get rid of the check engine light but then I might not see a serious problem if one arises (like a blinking CEL).

The only other way is to find the root cause of the problem. I suspect an exhaust leak since the guy at the used car lot said they put on a new exhaust (more like a new muffler, not an entire exhaust system). And by the looks of it they did a terrible job. It just might need a new gasket or there might be a small leak somewhere else. I'm not sure. My mechanic buddy did say it looked like they replaced the old 02 sensor so that's one thing I can check off the list. Again, if I can't track down the problem or if it winds up needing a new cat I'm just going to get an 02 sensor extension.

Does anyone have any experience with the 02 extensions?

 

Have you looked underneath your van to see if they indeed put a new oxygen sensor on, and whether it's the original large, downstream catalytic converter?

Without you having scanned the ECU for its readiness monitors and any pending codes, you don't know its status during the emissions test, unless you were given the results (a printout of them). Do you have these? Generally, though, you fail emissions, if a pending code is present, but that can vary from state to state (you didn't mention your state, so we can't look it up).

Note too, the recommendation by Toyota for fuel octane, for the 2002 Sienna. This may open a can of worms in this discussion, but this version of the 1MZ-FE engine can adjust timing to run on all grades of fuel, and perhaps run more efficiently with higher octane fuel. So premium fuel isn't a devil's elixir. But, fuel grade won't likely effect emissions a whole lot.

Bottom line: you're good for a while with your state, for emissions. So, get to know your van better, and consider purchasing a scan tool.

 

Putting an extension on the rear O2 sensor is not going to be easy. Various import tuners do this. However, the rear O2 sensor has a plate with 2 bolts that hold it down. It is not a screw-in type. So I don't see a good way to extend it. You might get a plate(maybe more than 1) cut to the size of the O2 sensor plate. But you would have to make a very large spacer to successfully move the O2 sensor out of the air flow very much. The import tuners buy a spark plug "anti-fouler" and drill it out. I think it has the same threads as the O2 bung. So you screw the O2 sensor into the anti-fouler, and then screw the anti-fouler into the exhaust bung.

Exhaust leaks around your muffler shouldn't be a problem. That is after the O2 sensor. Leaks around the connections to the exhaust manifolds and at the y-pipe to rear cat are the places you need to check for leaks. Use tissue paper on a rod to find the leak. The leaking air puffs will blow the paper around. If the front 2 manifold connections are removed and replaced, new gaskets should always be used. The gaskets there are "crush" gaskets. They may not seal a second time.

I fought the P0420 fight for a year before eliminating it. An updated ECU finally fixed it after I had thrown new aftermarket cats and new O2 sensor and new AF sensors at the problem. If you don't update your ECU, don't buy aftermarket cats. They won't pass the cat test for very long. They just don't have enough catalyst. Only OEM cats will pass the original ECU's catalyst test for any length of time. Also, if you buy cats you might only need the y-pipe with rear manifold pre-cat. A Toyota master tech told me that this pre-cat is the important one in the catalyst test. This jibes with what I've read elsewhere too.

 

I had a 2007 honda odyssey with 78k mi that had the same problem and gave me the same p0420 code as well. The check engine light would come on when my fuel was low or low octane it seemed, and when I pump fresh 89 or 91 the light would go away. For some reason it seemed to get along with 76 and Shell gas better than any of the other stations. My mechanic friend told me that a new cat would fix it. It wasn't that the cat was no good anymore, it was because the cat was so strict on emissions that even at 80% efficiency it would still consider that low enough to trigger the check engine light. This really sucked man, I too had bought it from a used car lot and drove it home before the light came on so I feel for you. I felt jipped but there was nothing I could do. So I just left it and drove it like that for 2.5 yrs. Recently sold it and got a new Sienna. Based on my experience, I wouldn't bother spending that $1500 or so to fix it. But if you feel you must replace the cat, I would suggest looking at an aftermarket one. I found one for the odyssey at autozone for $380 legal in California too. Otherwise, just leave it and drive.

 

RobertB - "An updated ECU finally fixed it". How did you go about doing that? Did you buy a new one? How much and from whom? Or did you have it programmed? How much and by whom? Our p0420 code comes on every once in a blue moon. Usually if it's really damp and cold. It probably hasn't been on in 4 or 5 months. I have replaced all the sensors before. Also, if replacing the oxygen sensor after the last catalytic converter, you might as well order new studs and nuts from Toyota. It won't cost much and will make your life much easier.

 

For my 2001, I bought a used 89661-08062 ECU from a local junkyard. It was the only one I could find in the country at the time, and it turned out to be only 20 minutes from my house! However, when I installed it, my van wouldn't start. So I went into a lengthy investigation to figure out that I needed to reprogram the 93C56 chip on the "new" ECU back to a "virgin" state. Evidently, it had keys programmed into it. My Sienna did not have an immobilizer... so it needs to have no keys programmed into this chip. This took me about 2 months to sort out and find the correct 256 byte bin to download into the chip. A new ECU will come in a "virgin" state already. However, you will pay over $1000 even online. I bought my used ECU for $120. The firmware of the ECU is not downloadable like many newer model ECUs. So you have to buy an updated ECU. There is a thread somewhere that documents what I did. I can probably find it for you if you can't locate it. If you search for 93C56 and ECU, that should find it.

The bolts on my rear O2 sensor were nice and clean. It helps to live around the Dallas area. If I lived in an area with lots of icy roads and salting, they would probably have been highly corroded.

What year is your Sienna? This ECU problem only affects 2001-03.

 

Our Sienna is an 02. And the code only pops up maybe two or three times a year so I don't think it's worth doing much about it unless it becomes a problem. But the information is good in case I need to do something. The roads in Indiana get salty. The nuts on the sensor mount had pretty much rusted away. I bought a stud remover from Summit auto and was able to unscrew the studs. I think I paid at most $8 for the two studs and nuts from the local Toyota dealer. That was probably 4 or 5 years ago I think.

I just had the left control arm and both outer tie rod ends replaced. It's very cold right now and I didn't want to deal with it but maybe I should have. It rides very good now but I looked underneath it since it's sunny and dry today. They kinda bent part of the frame where the back bushing bolt goes through. I don't know how much trouble they had getting that bolt out. I suspect they cut it out or something. Then on the right side, the boot that goes around the inner tie rod is torn. I can see it with a light. They didn't tell me. They could have just told me it tore and that I need to get it replaced. I understand these things. So I wonder how long I can go before having to replace it. It will require removing the outer tie rod end. Again, thanks for the info about the ECU.

 

mjpendrick, did you ever get the problem fixed? On my 2002, I had the same error code and Toyota quoted me $1500 to replace the cat. I ended up replacing both A/F sensors and it solved the problem. The one near the firewall was a PITA but after soaking it several times over a few days, I finally got it removed. I used an O2 thread chaser from O'Reilly's to screw back in the thread and it has worked fine since. I now have 223,000 on the car. I replaced them at 197K with two Denso AF sensors from Rock Auto. The back one was fouled out the worst. Hope this helps.

 

There's something I've thought of since I've worked on the Sienna, and the condition of your O2 sensors made me think of that.

The whole "hard to get to" thing of the rear bank in all these V6 engines has, from the start, made me wonder if it isn't a magnet for crooked car mechanics.

Example: family goes for ride across country in Sienna and check engine light comes on. They take it to a crooked gas station. The mechanic quotes them $1200 for replacing all six spark plugs and bank1 and bank2 O2 sensors. The family is happy when it only takes him an hour, they're on their way and the check engine light is off. The trouble is that he never touched the firewall spark plugs or O2 sensor. That's why it only took an hour.

I have a saying "if you can think of it, someone is probably doing it right now".

 

We had something similar with the rear spark plugs. The shop was doing other work and said with the rear plenum off they could go ahead and do the spark plugs and since it has been 90K miles since I last changed them, I thought that would be a good idea. I remember talking to the shop about which spark plugs to use. I said use the ones that are supposed to be used. I know they are pricey. The shop seemed weird about it. So flash ahead about 50K miles and I'm doing the rear valve cover replacement myself. I thought I'd go ahead and do the plugs since I'd have the whole thing apart anyway. Well low and behold, the three rear plugs are the same ones I put in when I bought the car used a long time ago. So these plugs have around 140K miles on them. So I look at the front plugs and they are the NGK G Power plugs and they are very worn out.

So what they did is charge me for 6 plugs (I have the receipt) and only changed the front three and put in the NGK G Power plugs to boot. Don't use those plugs, they are only a 30K mile type of plug. They looked really bad. Use the good stuff.

 

I'll bet that happens all the time. It makes me angry when it happens to me or another car guy, but what really ticks me off is that it happens to some single mom with a bunch of kids - that's actually evil.

I've attached a pic of a sticker under the hood of my 2000 Sienna (made in April). It says to only use the double electrode type of spark plugs. When I talked to the dealer here the plugs they tried to sell me were the single electrode type...

Most people will tell you to stick with the original Denso spark plugs and O2 sensors on account of the ECM being too rigid in what it accepts as ok. If you "experiment" with other brands of plugs or O2 sensors you are in unchartered territory that could likely result in the end of life as we know it.

 

So no dealerships would perform the TSBs to flash a s/w update. None.


I did the diode fix ($1 + $5 shipping) from RadioShack - worked like a champ!
No code and a passed emissions test!

 

No dealerships would perform a re-flash because the 98-03 ECUs are not flashable. That's why fixes require a new updated ECU. The 2004+ ECUs are re-flashable.

I like the diode fix. I never tried it myself. However, if I ever get the P0420 code again, I will definitely do it. It essentially just attenuates the signal from the O2 sensor to produce a signal that appeases the ECU.