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Toyotapro, ASE master tech
Category: Toyota
Satisfied Customers: 9974
Experience:  ASE Certified, 10+ years Lexus and Toyota Certified
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Toyota Avalon: good day.. 2000 avalon giving a P0420 code..

Customer Question

good day.. 2000 avalon giving a P0420 code.. how likely is it that the catalytic converter is actually bad.. and do you have to use a factory replacement. is it possible a MAF sensor could be going cause the the cat code to appear.. thanks
Submitted: 4 years ago.
Category: Toyota
Expert:  Amedee replied 4 years ago.

YES.. it is possible that MAF sensor is bad causing this code. The o2 sensor before and after the converter could also go bad causing this code as well. I have even seen vacuum leaks and exhaust leaks go bad causing this exact code. You are going to want to have the fuel trims checked with a scanner that can read the live data with the engine completely warmed up both at idle and under a load. They should read close to 0%. Both sort term and long term fuel trim. An allowable amount of up to 7% in either direction is ok.. but no more than 7%. If all of the numbers are good, no intake or vacuum leaks are detected and the MAF and oxygen sensors are all ok, then yes.. the converter needs to be replaced. YES.. it is important to use original equipment from the Toyota Dealership.

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Thanks in advance!

Best Regards

Expert:  Toyotapro replied 4 years ago.
Hello and thank you for using just answer!
A stand alone p0420 code is NOT going to be caused by a bad sensor on a late model toyota. Just an fyi. I do know a way around it to keep cost down but a factory replacement converter is the ultimate solution. New spark plugs should be installed also.
Expert:  Amedee replied 4 years ago.
Thanks Toyotapro but I have actually seen it.

You can also run a procedure that will wake the converter back up that just might help with this code. A trained professional should do this procedure for you.

Expert:  Toyotapro replied 4 years ago.
No, a bad sensor can lead to converter failure on these but it won't cause a 420 code to set stand alone. The computers are very sensitive on these and if something else is causing a lean/rich mixture such as a sticking o2 or air/fuel ratio sensor or maf sensor then it will also set a corresponding code such as p0171b172, 1135,etc......replace all the sensors in this that you want but it's not going to fix it. The only way to resolve it is to replace the cat with a factory one or install a blox adapter in front of the rear o2 sensor.
Customer: replied 4 years ago.
interested in ur way around it.. i have done many exhaust systems.. can i just buy a new cat and weld it in?
Expert:  Toyotapro replied 4 years ago.
My way around it doesn't involve a cat. If you weld any other catalytic converter other then a factory one in place I can tell you from experience it will still set a p0420 code. Install a fresh set of factory iridium spark plugs in the car first to be sure it's firing all the fuel properly. Then search BLOX on google, they make a set of mil eliminator adapters for imports especially honda and toyota. I have installed these on a few cars to offset the cost of a $1,200 converter and they have worked every time. It's about a $6 part and all it does is reduce the flow to the rear o2 that's after the cat to tell the computer that the converter is doing it's job properly. The computer recognizes low efficiency for a converter but it doesn't have a code for high effciency so this works beatifully.