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Ron Z.
Ron Z., - Toyota Tech -
Category: Toyota
Satisfied Customers: 18698
Experience:  18+yrs experience. State Inspector and Toyota Diagnostics
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2001 Toyota Sienna: P0420 OBD II code..code P0420..Catalyst System

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"P0420 OBD II code on 2001 Toyota Sienna. After checking code P0420 MOD $10, Catalyst System Efficiency Below Threshold Bank 1 listed 2 times."

Welcome to! My name is DriveFast71(Ron) and I will do my best to answer all of your questions completely and accurately.


The P0420 is a fairly straight forward code pointing to a fault with the Catalytic Converter. The Oxygen Sensors monitor the amount of emissions in the exhaust going into, and then out of the converter. When they see the converter is not 'cleaning" the exhaust enough to meet Toyota standards, the P0420 is set, and the converter is deemed "below threshold". Most commonly, this means the replacment of the converter. Best bet is to monitor the wave lengths of the oxygen sensors (this requires the use of a scan-tool) to make sure they are functioning properly. But, most commonly, this is straight forward, and the converter needs to be replaced.

Customer: replied 5 years ago.
I thank you for your answer. I have read that this is the probable cause but I also read that the engine coolant temperature sensor is possibly not reading right. But to be truthful the car is not running hot according to the gauge inside the car. If I were to get a relatively good catalytic converter, what OEM part would you suggest?

When the ECT (engine coolant temp) sensor is faulty, it will set the appropriate code for this (P0125). The Toyota OBDII system is really a tight system, and when something is faulty, it pinpoints it, and sets the appropriate p-code.


When replacing the converter, you'll need to get as close to Toyota OE specs as possible. Those "universal" or "cheap" converters they sell at AutoZone will NOT solve the problem. I've had fairly good results with "Walker" and "Bosal" brands. Try to use a "better quality" parts store like NAPA. On this vehicle, the converter is built into the Front Pipe, and the Toyota OEM Part# XXXXX: 174100A310

Ron Z. and 3 other Toyota Specialists are ready to help you
Customer: replied 5 years ago.
How many converters does the Sienna 2001 have?
"Technically" it has 3. There are 2 "warm-up" converters, one for each bank. These are part of the Exhaust Manifolds. And there is a "main" converter that is located under the vehicle, in-line with the exhaust system. The P0420 is most commonly a faulty Bank1 "warm-up" converter. This is the side of the engine closest to the firewall. It will need to be replaced as an exhaust manifold/converter assembly. The Toyota diagnostic flow chart suggests that the Bank1 and Main converter should be replaced with the P0420, but commonly, just the warm-up usually is the culprit. Proper diagnostics should always be performed to confirm this.
Customer: replied 5 years ago.
Thanks. I have turned off the check engine light 1x but when it came back I did not check for any additional to the P0420 readout. I can check now. I get a slight hollow sound as if the muffler has a hole in it. Is this the usual?
Sometimes when the converter is faulty, there will be a change in the sound of the exhaust, yes. Like I say, the Toyota on-board diagnostics are extremely tight. If the P0420 has returned, the converter is "on it's way out".
Customer: replied 5 years ago.
I could check for codes again if you wait about 10 minutes, okay?
Not a problem. I'll be here...
Customer: replied 5 years ago.
Thanks for your patience. I will give you the information pretty much how it is setup in the scanner.

IM Mon since DTC cleared
P0136 O2 Sensor
Circuit Bank 1, Senor 2 (see P2270, P2271)
Pending Global Codes 2
(Catalyst Mon Inc) is also mentioned.
The P0136 is pointing to a fault in the Oxygen Sensor located behind the faulty converter in question. If the Catalyst code has returned, this code could be a byproduct of the faulty converter which is fouling out the O2 sensor. Unfortunately, without the use of a scan tool, you cannot test to see if there is an electrical problem with the sensor, or if it truely is being fouled out by the converter (which does happen when the converter goes bad, and is not addressed. The faulty converter can quickly damage oxygen sensors).
Customer: replied 5 years ago.
It did not give me the P0420 code but it gave me these 3 codes P0136 and the P2270 and the P2271 which I have seen before dealing with the 02 sensors. Does this possibly mean that the P0420 is still there just the scanner does not read it because I cleared it?
The P2270/2271 are not valid codes for this vehicle. I'm going to take a guess and say the code reader you are using is a "generic" code reader, and when it said "see P2270, P2271" this was for diagnostic purposes.

It's quite possible the vehicle hasn't had enough miles put on it yet for the P0420 to reappear yet. Or, the converter is "on it's way out" and hasn't shown the on-board computer symptoms that it did to set the P0420 in the first place.
Customer: replied 5 years ago.
I have tried not to drive the car as much. So you think it is only the P0420 code?
Only way to tell is to put some miles on the vehicle. If the converter is the actual culprit (which is a very common Toyota fail item), the P0420 will return. If the P0420 was the very first code that came up, and now you have a failing O2 sensor, it's a good bet the converter is the cause.
Customer: replied 5 years ago.
So should I purchase an O2 Sensor just in case and which one does the P0136 indicate?
If you'd like to try replacing the O2 sensor first, you can. Without seeing any diagnostic info on the sensor with a scan-tool, i cannot confirm this will solve the problem. It's located under the vehicle, behind the converter shown in the image i sent above:
Ron Z. and 3 other Toyota Specialists are ready to help you
Customer: replied 5 years ago.
Thanks for ALL your help.
You're welcome! Have a great day!
Customer: replied 5 years ago.
Thanks so much!
Have a great day!