Question about 2002 Toyota Sienna

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Just replaced O2 sensor a few weeks ago and check engine light has returned for yet a second time. Code is now reading P4020 Catalyst efficency below threshold.

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Sounds like the converter is bad. There is also a service bulletin from Toyota for this problem. They have released new computers to be installed that are less strict on how well the converter is working.

Posted on Nov 11, 2010

Testimonial: "Thank you for the information!"

  • rickner99 Nov 11, 2010

    could you tell me about the service bulletin or where I could read about it? also any idea on the estimated cost ?

  • Clint Smock
    Clint Smock Nov 12, 2010

    When I get home tonight I'll try to post the bulletin. Give me your VIN and I'll see if you van is listed under the bulletin.



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There is no such code for any car. this 1999 cougar with the 2.5 engine has a code and i think you may have mixed the numbers the only code thats close is the po420.

Code po 420 is the cataylic converter efficency, this means the converter is not cleaning the exhaust to specs anymore and may be bad.

I have found that i lot of times a bad o2 sensor is the cause of this or both o2 sensors as in this car,i have found that its good thing when a converter is replaced that the o2 sensors should always be replaced. this will cost more but is the correct way to do the job. what i do is replace the o2 sensors first and then reset the light and then if light doesnt come back problem is fixxed.
there are mant people out there that say a good tech can see if the sensors are bad, ture and not,you can test a sensor by watching the reading and also changing them to see how fast a sensor can react and theres more to it but i wont bore you.

MY point is , i have replaced bad o2 sensors and then the converter is bad, now the customer is not happy as the quote just went up alot. then i replace a bad converter and then find an o2 that worked a week ago is now slowed down,
This is to keep in mind if its a converter and the o2 sensors are old replace them, they may be working now but lets say they slow down but not enough to turn engine light on which i see alot, then a year later you wonder why you must buy another converter.

Good day.

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Catalyst system efficiency below threshold

is that the code you get from comp is PO420 eng light if so worst news either you have an exh leak between o2 sensor 1 and o2 sensor 2 or the front o2 sensor is lazy or the front catyletic converter is not working

basicaly code is only triggerd when engine is hot and the front o2 sensor and the rear o2 sensor read the same voltage when conv is working the front o2 sensor will fluctuate rapidly from 0v to 1v and rear o2 sensor will remain steady at 0.5v

try to explain best i can plz vote comment or reply if you need more help

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you may have a bad converter, its not cleaning the exhaust, but on the other hand a bad rear o2 sensor can also cause this problem,there is a wat to test but you would need a scan tool or scope,replace the other sensor, if light returns then replace the converter, no money lost, if you were to replace the converter the o2 sensors should also be replaced. you have 165000 miles it would be crazy not to replace sensor and install new cat, so basically your just doing it in 2 steps and may get lucky and not need a cat. good luck.

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