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Discussion Starter · #1 ·
So I just purchased a 2006 Tacoma 4x4 with the 4.0L. The truck was too quiet for me so we added a Flowmaster 40 series in place of the factory muffler. Still too quiet - so we removed the second cat or premuffler (not sure which it was) on both sides that were located after the last o2 sensors. Now a couple weeks later I am getting a check engine light with the P0420 code. The only thing I can't understand is that although we removed the second cat/premufflers they were located after the o2 sensors so how could the computer now be throwing a code? Any help would be great I hate having the CEL light on the dash.
 

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06 Taco 4x4 TRD
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I don't know, your logic seems right, but if you're only getting one code, that's just one side (right side I think.) Did you ding the o2 sensor when you cut off the cat?
 

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GO KING'S GO!
05' Taco Pre-Runner
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So I just purchased a 2006 Tacoma 4x4 with the 4.0L. The truck was too quiet for me so we added a Flowmaster 40 series in place of the factory muffler. Still too quiet - so we removed the second cat or premuffler (not sure which it was) on both sides that were located after the last o2 sensors. Now a couple weeks later I am getting a check engine light with the P0420 code. The only thing I can't understand is that although we removed the second cat/premufflers they were located after the o2 sensors so how could the computer now be throwing a code? Any help would be great I hate having the CEL light on the dash.
^ Is that the 40 Delta Flow? Sounds like you may have hit the sensor or the truck is learning it's new setup? I would try and clear the code and see what happens!
 

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Here's an explanation of that code from the Service Manual.

The ECM uses sensors mounted in front of and behind the Three-Way Catalytic Converter (TWC) to monitor its efficiency. The first sensor, the Air-Fuel Ratio (A/F) sensor, sends pre-catalyst information to the ECM. The second sensor, the Heated Oxygen (HO2) sensor, sends post-catalyst information to the ECM.
In order to detect any deterioration in the TWC, the ECM calculates the Oxygen Storage Capacity (OSC) of the TWC. This calculation is based on the voltage output of the HO2 sensor while performing active airfuel ratio control, rather than the conventional detecting method, which uses the locus ratio.
The OSC value is an indication of the oxygen storage capacity of the TWC. When the vehicle is being driven with a warm engine, active air-fuel ratio control is performed for approximately 15 to 20 seconds. When it is performed, the ECM deliberately sets the air-fuel ratio to lean or rich levels. If a rich-lean cycle of the HO2 sensor is long, the OSC becomes greater. There is a direct correlation between the OSCs of the HO2 sensor and the TWC.
The ECM uses the OSC value to determine the state of the TWC. If any deterioration has occurred, it illuminates the MIL and sets the DTC.​

The manual goes on to say that the primary causes of this code are (in order):
- Exhaust Leak
- Sensor Problem (either A/F or HO2)
- Catalytic Converter Problem

I think in this particular case, assuming a sensor isn't damaged, the new exhaust system is allowing so much airflow as to make the computer detect a decrease in the efficiency of the catalytic converter.

I'm not sure how you would fix that. Maybe someone else has run into this before.

 

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Discussion Starter · #5 ·
Some more info. on my truck in this post. It only has 34k miles on it. I'm guessing a factory cat would still be good at this low of mileage - right? Also, I have reset the computer several times by disconnecting the battery and the light manages to keep coming on; sometimes after 20 minutes of driving and other times after a week. I have checked for leaks also as the y pipe was removed to cut the cats off and don't seem to find any leaks. I pulled the right side post-cat o2 sensor out today and it looks ok, however when the exhaust was welded up this sensor was used as ground - could that damage the sensor? In addition, if it was a flow issue wouldn't it throw a separate code for bank #2 (left side) since both sides are set up the same? Anyone else having similar problems?
 

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06 Taco 4x4 TRD
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That's what I was thinking about the flow issue. One side only. The sensors are pretty robust but I guess you could have fried it on welding. One idea would be to swap the sensors side to side, and see if you throw the 0430 code. That would point to a bad sensor. If you repeated the 0420, I'm out.
 

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Discussion Starter · #7 ·
So I switched the rear o2 sensors side to side hoping that my truck would throw the P0430 code meaning that I had a bad right rear sensor. No luck! Still throwing the P0420 (right side bank 1 code). Now I'm lost! Either the sensors are seeing too much airflow due to the second cat being removed or I have a bad pre-cat sensor or main cat only on the right side. Could a cat or sensor really be bad at 35k miles? I hate the damn CEL being on! Any help would be greatly appreciated. Anyone experienced a CEL after exhaust work? Thanks!
 

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2006 Tacoma 4x4
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Hey JD, I've had this same problem except i havent done any exhaust work. if you have an OBDII reader hook it up, go to live data scroll down to the s2b1 V if its fluctuating up and down a lot more than s2b2 your cat is gone, if the V is way off s2b2 which we assume is ok cause its not throwing a code then its the sensor. regardless arent you under warranty? i was, they wanted to charge me $75 for the diagnosis but i argued them to fix it all under warranty. the cat is part of the header so they replaced the header down to the joint by the y-pipe. now 10 000kms later i get code p0430 no more warranty:cursin: i have bone stock exhaust!
 

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Discussion Starter · #9 ·
Yes the truck is still under the factory 36k mile warranty; I believe, however I don't know if they will touch it since I removed the second cats.
 

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2006 Tacoma 4x4
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I'd take it in anyway, the exhaust has a longer warranty than the drivetrain 130000kms (80000 miles) just argue with them that what you did was after the cats so it has no effect on it. pretty much a cat back exhaust. good luck with the dealer.
I'm not too sure about this too much flow theory, it cant flow that much more than before because the engine is making the same amount of exhaust, it can flow more freely when you stomp on it, less resistance, but the amount would be the same. it would be whatever the the combustion chamber size is, that wouldnt change.
 

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Discussion Starter · #11 ·
So I plan on making an appointment at the dealership next week and I'm hoping I won't get screwed over! They told me that removing the second cat shouldn't effect the computer or sensors in any way, however if they find that the modifications I did to the exhaust are causing the issue it will not be covered; more money out of my empty pocket! I'm sure they will find a way to blame the issue on me! In case this happens, is there an easy way to unplug the CEL out of the dash so I won't see it anymore?
 

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Discussion Starter · #12 ·
Update: I took my Tacoma into the dealership today for the diagnostic hoping for good news! A couple hours later I get a phone call from the Service Writer telling me that I'm missing the rear cats and that it's going to cost $3,600 for a complete new exhaust. I said, "No shit the cats are missing and the CEL is on! That's why I brought it to you!" After telling him that the computer shouldn't be seeing that the rear cats are missing and that I'm only getting a bank 1 (right side code) and not a bank 2 (left side code) - which it should throw if it was the missing cats and that was the issue. He said he would have the tech who worked on it give me a call back. Two hours later the tech calls and says that the light is coming on because of the missing cats. I asked him to explain how the computer would see this. He couldn't give me and answer and then went on to say that the upper cat is failing also and this could be causing the light. Which is it the missing cat or a bad right side upper cat? I love it when they change their stories under pressure. It's sad when you know more than the Toyota tech working on your truck. Anyway, after a couple more phone calls and some more arguing with them, next week I'll be getting a new right side manifold with upper cat installed under warranty. You'd think after buying a fairly expensive pickup from them that the dealership would be more willing to help. So we'll see next week, if the upper cat truly is bad this should solve my P0420.
 

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Just pin it, Ya' fairy!
08 tacoma
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no matter how expensive you vehicle dealers don't like giving stuff away, my dad bought a mercedes ml63 amg which is 120,000 cad and had some tranny problems and they wanted to charge him labour for it even though it was cleary under waranty.
 

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Discussion Starter · #15 ·
Last Tuesday I had the right side manifold with upper cat replaced under warranty. It's been a week and no P0420 code! I just can't believe that a factory cat would go bad at 35k miles. Oh well; gotta love warranty work!
 

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Converters don't just go bad. There has to be a reason why the code was set. There is absolutely NO reason for a converter to go bad on it's own after just 34k miles. I sell a ton of converters through my store and I always end up getting a phone call from the shop I sold one to a few weeks later saying that I sold them a junk converter. As per usual, I send the cat back to the manufacturer for testing to see what when wrong. 99 times out of 100, I get the test results back and they show that there was excessive heat, unburned fuel, coolant, excessive carbon build up, etc. There is something going on in the engine to cause that P0420 code. Every distributor that I deal with got so many "defective" returns for Catalyst below efficiency codes that they stopped warrantying the converters for the code is present and started testing EVERY converter sent back to them.
It is the same thing with Nissan V-6 knock sensor codes. I sell a ton of knock sensors because technicians don't know how to use scan tools to find real problems. They call me for the knock sensor then a week later they call back to tell me that I sold them junk parts. There are no real technicians left anymore. I'm sure of it. All these parts stores with their super liberal return pollicies have turned the industry into a bunch of parts changers. Dealerships are no different than any other shop.
 
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