Po420 error code buick lesaber (2003)

DAN1970

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Hi

I would really appreciate a little input. I changed both 02 sensor on my Buick LeSabre (2003) after seeing engine light appear in dash (PO420 code). I erased the code after changing the sensors, but after doing about 50 miles the check engine light re-appeared and the code is once again PO420. Do any of you think it would be a converter issue? If yes, would that give me issues will the motor down shifting when its not suppose too every once in a while?
Thanks for the input!
 
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redmondjp

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It's hard to say - that code is for catalytic converter low efficiency, so it could very well be the converter. It can also be caused by O2 sensors - did you use OEM type sensors or aftermarket ones?

I would recommend taking the car to a shop that is good at driveability issues and emissions testing/repair, before throwing a converter at it (those are pretty expensive).

How many miles are on the car?

And what is your other issue - it downshifts when it shouldn't?
 

DAN1970

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Both 02's are aftermarket. It seams to downshift out of nowhere for no reason going on a straight way with no incline. It has 128,000 miles.
 

dcraig1000

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oem is always the best to use for most parts, but based on my experience with this code, a new cat always has fixed the problem.
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AprilA

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oem is always the best to use for most parts, but based on my experience with this code, a new cat always has fixed the problem.
I have this code on my 17 encore also had 2 Catalytic converter put on it a new
Flex pipe,new o2 sensor bucik cant seem to find the problem
 

2004CSOTP

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Replacing the catalytic converter fixed this for me. I have an 04 Lesabre with about 120,000 miles on it.
 

2007LucerneCXL

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This is the Lesabre forum completely different engine and vehicle not compatible on problem.
 

broke_collegekid

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Yeah my exhaust currently smells horrible (Assuming that is NOX), so basically I am saving up to get a catalytic converter replacement.

Any one have recommendations on a brand and how much I should expect to pay for labor?
 

broke_collegekid

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Usually a rotten egg smell is due to a rich mixture (unburnt fuel). Are there any codes or other issues?
Well I got P0420 and it hasn't gone away despite trying some fuel injector cleaner, and cataclean treatment as well. Exhaust kinda just smells horrible and strong - not really that sulphur type smell that I smelled at yellowstone and in my dishwasher when it got clogged. So I'm thinking the converter isn't burning off all the NOX or whatever other gases it's supposed to break down. A shop checked for exhaust leaks and didn't find any, said it could be that the cat just went bad. Car at 185k miles and I did plugs & wires at 175k. I also did the diagnostic test found online with plugging in the OBD-2 reader that can read data in real time and running the car. Upstream O2 was reading in a normal way but downstream was really erratic which indicates bad cat.

Here's the video I watched on diagnosing

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LARRY70GS

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I think it is very unusual that the exhaust smells bad. Misfires that would overheat the cat should also pop a misfire code. In any case, using an O2 sensor extender at the rear sensor has fixed many a P0420 and P0430 code. It works by giving the sensor less of a sample.

 

Mad_Coachman

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I think it is very unusual that the exhaust smells bad. Misfires that would overheat the cat should also pop a misfire code. In any case, using an O2 sensor extender at the rear sensor has fixed many a P0420 and P0430 code. It works by giving the sensor less of a sample.

I used an L-shaped adapter from Big Daddy's Garage. Google them.
 

HotZ28

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I used an L-shaped adapter from Big Daddy's Garage. Google them.
The downstream adapter, is nothing but a band-aid fix for the real problem.
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LARRY70GS

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The downstream adapter, is nothing but a band-aid fix for the real problem.
Yes, but what is the real problem? One of two things, either the engine is worn/leaky enough that it is burning oil/coolant and fouling the catalytic converters, or, the replacement aftermarket cats are using less precious metals and not getting the job done according to the PCM. In any case, if the dealership isn't covering the cost of the repair, and it gets expensive, that 6.00 spacer starts looking awfully good.
 

AprilA

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Yes, but what is the real problem? One of two things, either the engine is worn/leaky enough that it is burning oil/coolant and fouling the catalytic converters, or, the replacement aftermarket cats are using less precious metals and not getting the job done according to the PCM. In any case, if the dealership isn't covering the cost of the repair, and it gets expensive, that 6.00 spacer starts looking awfully good.
I wonder if that spacer would fix my problem,, as we know forsure its not the Catalytic converter after 2 being replaced, and sensor changed and flex pipe fixed, new battery, new coil pack new spark plugs, new manifold cover and many trips to buick, but code po420 still up .. u think it would have something to do with my injectors?
 

LARRY70GS

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It won't cost you much to find out, and if you can replace a spark plug, you can do it yourself. If the engine was running too rich or too lean, the PCM would detect it and store other trouble codes.
 

HotZ28

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To the OP! The most common failure of the P0420 code on the 3800 engine, would be the cat after 100k miles, and more than likely, contributed to the valve seals leaking oil or, coolant contaminating the cat!
 
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