p0420 IN 2000 SIENNA

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Jan 21, 2012
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No. California
The husband of a friend of my wife was over today for a BBQ and related to me a problem with his 238,000 mile 2000 Sienna. Now he almost zero knowledge on cars and I had to show him how to do an oil change on a BMW 538i. So this Sienna threw a P0171 code a week ago. First guy was a mobile mechanic who said it was the MAP sensor. Yeah, right. Yet he had the MAP sensor replaced. Runs Ok but of course a code shows up later after running and resetting. Brings in to Toyota Pro (independent) who tell him he has a problem with bank 2 sensor 1 now. They replaced the sensor and guess what. A day later a code is thrown again. Brings back to them and tell him he has another problem with, you guessed it, the MAP sensor. They did check his engine out for any vacuum leaks using smoke.

Getting good. So he buys another new MAP sensor and has another (third) mechanic install it. Once again a few days later the next new code is P0430 which he got because he borrowed a scan tool. Calls the same mobile mechanic says it is an air/fuel sensor. He gets that replaced. After driving 56 miles the fourth code is thrown. P0420 which he check with the scan tool. He then goes out and buys a new sensor for downstream now. It is installed. Code is erased but of course comes back. Goes to a muffler shop and they say he needs a new converter immediately. Maybe so but they did no testing to prove their diagnosis correct. Easy enough to check the output of the O2 sensors when hot and running at 2000 rpm. Also easy enough to check the inlet and oulet temperatures of the converters.

Now it is quite possible it could need a new converter since almost everything else was replaced by throwing stuff at the car. The engine itself is running fine except for the CEL. I do my own work and am not about to deal with this as I have 10 cars to deal with. Just irritates me that mechanics jump to all sorts of conclusions and I can't recommend a good mechanic. Being a doctor I have a procedure I have to follow when a condition is presented and I have other possibilities in the differential diagnosis unless the condition is blatantly obvious like you have a nail in your eye so no wonder it hurts. Anyway he is wondering who to go to next but he now knows almost as much as I do on the subject as he is right here. Most likely a new cat but at least someone who can prove their diagnosis. LOL, I told him to quiz mechanics now... that should be fun.
 
Joined
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Time for new mechanics. As you say a simple temperature difference test between the inlet and outlet of the converter will provide the answer as to whether the converter is working or not. The O2 sensor pattern on a scope or scantool can also be used, the rear O2 pattern will begin to mirror the front O2 instead of a steady reading.
Using an IR thermometer there should be about 70-150f difference with the engine at normal operating temp.
An exhaust gas analyzer can also be used to determine cat functionality. Engine vacuum test and exhaust back pressure test can identify a plugged cat.
 
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tbm3fan

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Jan 21, 2012
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Time for new mechanics.
Easier said than done as he is unlikely to trust one now. First his BMW and now this car with me solving his BMW some years ago after mechanics threw a thousand dollars worth of ignition parts at the car that did not have an ignition problem to start.
 
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I think you mean MAF (Mass Airflow Sensor)......Low engine load values at Wide Open Throttle is a good indicator that the MAF could be faulty. It's curious that it will set a lean code for just 1 bank & not both banks....But I've seen several V6 Toyota/Lexus vehicles do just that.

I don't believe a 2000 Sienna has "Dual Catalyst Monitoring"......Has 2 Pre-Cat's in the Y-pipe that feed into a Primary Catalyst & has a single post cat O2 Sensor. So I don't believe P0430 is a plausible DTC. P0420 can set if one or both Pre-Cat's fail OR the Primary Catalyst has failed.

Sure.....There are a couple things that can cause a P0420 other than the Catalyst's....Namely Exhaust Leaks & O2 Sensor Performance.

Not saying that you or your buddy in particular are this way....But the vast majority of the motoring public don't want to pay for Diagnostic time.

I was a GM tech for many-many years before expanding into general automotive, One thing I've noticed about Toyota's is their extreme low tolerance for catalyst efficiency deterioration....I've had some pass Nox, HC, CO, & Oxygen Storage testing, Reflash the ECM & still set Catalyst Efficiency DTC's, Only new (Sometimes OE) Catalyst would fix them.
 
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Austin, TX
P420 and P430 are the most notoriously overloaded codes - meaning there is nothing in the manual to point to the root cause of the problem.
P420 is nasty code to deal with and almost invariably it is the O2 sensors (past cat) and AF sensors (pre-cat). Start there bank2 pre-cats next to the radiator and then the bank1 pre-cats are a pain to get out and it is labor expensive.

All in all time to get rid of the vehicle is a good option because it is a CA vehicle - emissions are notoriously strict. But hey all good things comes out of CA - we won't have a Tesla if it were left to Feds to write up emission levels (feds are easily 20 yrs behind CA in emissions, they are even such low lives they copy over CA emission standards and call it Fed standards)
 

JC1

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Nov 29, 2008
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Oshawa, Ontario Canada
I had a 98 sienna and I'd get a p0420 periodically. With the scan tool I would reset it and it wouldn't come back for months.

I replaced the cat with a cheaper aftermarket cat every 3-4 years. The OEM version was $1150, so it was cheaper to buy a $200-250 a few times.

Try to reset it and see of it stays off for a while.
 
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