P0420

mstrsail

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Hi, I have a 2005 Buick Lacrosse with the 3.8 engine and it has 55, 000 miles on it. I had a P0420 code come up. The garage has been trying to fix it for the last five months. They have replaced the Cat 4 times with different brands. The O2 sensors twice. New plugs and wires and a couple new fuel injectors. The code still comes up. Sometimes only after 70 miles and it went as far as 500 miles. Always the same code. When I plugged in a scanner and looked at the Freeze frame data it seems that when the code is triggered the car is at idle or near idle and not when it is going down the highway. The garage even took it to a GM dealer twice. The first time they said it passed all tests and the second time they said that I need a genuine GM factory converter at an outrageous price of almost $1500 to fix it.
Could this be true and why can they sell you aftermarket cat converters that will not work?
Any help with this situation would be appreciated.
 

mstrsail

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Thanks for the info. Yes, they said they did and they replaced it four times from different manufacturers. It is just amazing to me that you have to buy a $1500 cat from a dealer on a 13 year old car to get that code to go off. You would think they would let you reprogram the ecu for aftermarket cats. I have already spend that much for the diagnosis and the parts they have replaced with no improvement in anything at all.
 
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#4
I would just put on an aftermarket cat and remove the P0420 code from the PCM. They are way too sensitive from the factory on this stuff, and since you are out of the extended emissions warranty, it's not worth all of that extra cash for a genuine cat. Yes they are expensive, but they are built way more restrictive.
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mstrsail

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I would just put on an aftermarket cat and remove the P0420 code from the PCM. They are way too sensitive from the factory on this stuff, and since you are out of the extended emissions warranty, it's not worth all of that extra cash for a genuine cat. Yes they are expensive, but they are built way more restrictive.
How would I go about removing the code from the PCM? The car runs fine and I do really not want to put anymore cash into this code.
 
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#6
How would I go about removing the code from the PCM? The car runs fine and I do really not want to put anymore cash into this code.
Look into someone who does PCM tuning. There are tons of people that do it for the 3800. If you live near Southeast PA, I can even do it for you. I just don't do anything through the mail anymore, I prefer to do it from the passenger seat.
 

mstrsail

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Look into someone who does PCM tuning. There are tons of people that do it for the 3800. If you live near Southeast PA, I can even do it for you. I just don't do anything through the mail anymore, I prefer to do it from the passenger seat.
Thank you, I will check around here. I am in southeastern, CT. I can live with the light but when the emissions come due in two years I will want it off. It did pass with the first aftermarket cat they put on but the light came on again about 50 miles later. Since then they have put on three more and this last one lasted around 500 miles before the light returned. Not sure if I turn it off if it will pass again but I will give it a try if I can not find someone to delete the code.
 
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#8
Since then they have put on three more and this last one lasted around 500 miles before the light returned. Not sure if I turn it off if it will pass again but I will give it a try if I can not find someone to delete the code.
That's just the car running it's self-diagnosis. Nothing wrong with the cat, it's just looking for way too strict of emissions. If you turn it off with a code-reader and try to get it inspected, the car will probably report that the emissions system is not-ready and they'll make you come back until it reads a ready state. If you delete the code from the PCM, you'll pass every time.

You could probably even just sign up on the HPTuner forum and see if someone is local to you. It would be 2 credits to hook your car up to their HPTuner ($100), and then whatever the person wants for their time. I usually just charge people $125 for the whole thing to help them out.
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#9
That's just the car running it's self-diagnosis. Nothing wrong with the cat, it's just looking for way too strict of emissions. If you turn it off with a code-reader and try to get it inspected, the car will probably report that the emissions system is not-ready and they'll make you come back until it reads a ready state. If you delete the code from the PCM, you'll pass every time.

You could probably even just sign up on the HPTuner forum and see if someone is local to you. It would be 2 credits to hook your car up to their HPTuner ($100), and then whatever the person wants for their time. I usually just charge people $125 for the whole thing to help them out.

Deleting the code from the ecm might still have it as a not ready parameter for the emissions readout if the 02 is not working properly or if the 02 heater sensor has been removed all together... but cars are allowed 1 not ready monitor and still pass emissions.
 
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#11
Good question, and when culprit is changed, no results. Has to be something else related but not explored yet. O2 sensor would seem likely as brought up also. For some reason am thinking of PCV leak or something like missing gas cap or not sealing. Would seem though a dirty or misfit toilet (that came up, spell check lmao) MAF sensor rather may be connected?
 

mstrsail

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Good question, and when culprit is changed, no results. Has to be something else related but not explored yet. O2 sensor would seem likely as brought up also. For some reason am thinking of PCV leak or something like missing gas cap or not sealing. Would seem though a dirty or misfit toilet (that came up, spell check lmao) MAF sensor rather may be connected?
I also thought that it may be something else but the garage has taken it to the dealer twice to get it diagnosed and the last time they said it needed a factory cat. They did replace both O2 sensors twice.
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MelsRegal

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#13
did the shop go through every item in the list below?
what manufacturer and part number Cat is currently in the car?

Leaded fuel was used where unleaded was called for (unlikely)
A damaged or failed oxygen / O2 sensor
Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
Failed or underperforming catalytic converter
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings
Leaking fuel injector or high fuel pressure
Cylinder misfire
Oil contamination
 

mstrsail

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did the shop go through every item in the list below?
what manufacturer and part number Cat is currently in the car?

Leaded fuel was used where unleaded was called for (unlikely)
A damaged or failed oxygen / O2 sensor
Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
Failed or underperforming catalytic converter
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings
Leaking fuel injector or high fuel pressure
Cylinder misfire
Oil contamination
They just told me that they checked everything and that the dealer did the same. I know they got one cat from Napa and they also got two from walker and the last one is a Bosal from advance auto. The last one has lasted the longest, about 500 miles before the light came back on. I have read all the info on the code and told them about it but they really don't want to hear a customer telling them how they should do their job :) This repair was done at AAA repair facility and they are suppose to guarantee the work for 2yrs or 24000 miles. But after $1500 spent the car runs no better and the light still comes on, even after they replaced all those parts. The AAA person I am in contact with is trying to help but from what he was told by another dealer it that a genuine GM cat is needed. I think another $1500 for that cat is too much to pay, and there is really not much I can do to get the money that I spent back. This have been going on since Jan. 20th of this year and I know they do not have the old parts that they replaced anymore so they can say whatever they want the the AAA person.
 

MelsRegal

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#16
is the shop willing to install a factory cat for free labor or a reduced rate?
you can buy a factory cat, 15861400 for less than 1k on the internet.
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mstrsail

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is the shop willing to install a factory cat for free labor or a reduced rate?
you can buy a factory cat, 15861400 for less than 1k on the internet.
The AAA person is checking on that now. The problem is if they replace it with a gm cat and the light comes back on I want all that money refunded and if I bought the converter off the internet, I may not be able to return it.
 

mstrsail

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They have replaced the cat for the 5th time. This time they put on a Calif emissions cat. They just found out that my car has the calif emissions sticker under the hood. The dealer didn't even catch that. The car went 1300 miles and I got the code again. I have talked with a dealer and asked if I put on the factory converter that if I got that code again would they put back on the old one and refund my money and they told me no. Everyone is telling me that there is something else going on and that replacing the converters and o2 sensors all this time is telling them it is not those parts. The freeze frame data indicates that this happens at idle or stopped and not going down the road.
 

mstrsail

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I had a zzp downpipe with magnaflow high flow car on 2 different 3800 cars and had no problems.
This last one was a magnaflow 456029 and it lasted the longest before the code came on again. Next week I will have it diagnoised somewhere else and go from there. A couple GM dealers I have talked with said I shouldn't have to use a factory $1500 cat but a couple others said I might have to. Either way it has been nearly 8 months and numerous parts replaced and the code still comes up :)
 
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