Question about 2005 Chevrolet Avalanche

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I put headers and a new y-pipe on my 05 avelanche and a code (p0420) came up. Would replacing the o2 sensors take care of the code? I need to take care of the problem to get the vehicle inspected. I'm stationed at Ft. Bliss and can't get post decals with out the inspection. I rebuilt the transmission about 2 weeks ago and now its something else. There is no warrenty so I do all the maintenance myself.

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There's no guarantee that it will cure your code p0420. It's dependent on back pressure and factory settings vs the exhaust package you put on. Is there a possible waiver you can get from base? Based on vehicles age you might have that option. I'm not that familiar with military emission rules, but lots of states offer a waiver option. Wouldn't hurt to ask before throwing a handful of parts at it.

Posted on Jan 04, 2011

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Posted on Jan 02, 2017

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Code p0420 acura m'dx


1996-newer (OBD-II) vehicles use oxygen sensors and a catalytic converter to reduce emissions. There is one O2 (oxygen) sensor in front of the catalytic converter (called upstream), and one behind it (downstream). When the engine is warmed up and running normally the upstream oxygen sensor reading fluctuates and the downstream one is fairly steady. In the case where both readings are fairly steady, a P0420 check engine light code is set.
Possible Causes: Oxygen (O2) sensor not working correctly
Damaged or leaking exhuast (pipe, manifold, catalytic converter)
Damaged catalytic converter
Diagnostic Help:
To diagnose a P0420 trouble code we begin by checking for exhaust leaks under the vehicle. Then use an oscilliscope to check the waveforms of both upstream and downstream O2 sensors. The front reading should vary, the rear should be steady. Replace as necessary. Check the rear Heated O2 Sensor. Check the catalytic converter, replace if necessary. If necessary, check for wiring and component concerns. See our article "Automotive Circuit Testing 101", if you need more assistance with this. Remember to refer to an model-specific repair manual for proper instruction.

Jan 20, 2014 | 2005 Acura MDX

2 Answers

2005 Nissan Altima code P0420


code p0420 refers to the cat converter system-bank 1--efficiency below threshold===causes --wiring--cat converter -- HO2S 2. I would replace the heated oxygen sensor number 2. There is no real life on cat converters as they are extremely fragile and run at extreme temperatures which means that if you hit a icy puddle of water / snow and the cat is hot they are prone to cracking and disintergrating internally and blocking up the exhaust pipe.. In your case I would be looking at the sensor. Use the correct tool so that the wires are no damaged.

Jan 05, 2014 | 2012 Nissan Altima 2.5 Sedan 2.5L 4-cyl....

1 Answer

Replaced catalyst converter, both sensors and still have code P0420 what's the problem


Hi there:
The possible causes of this code are: exhaust leak between the exhaust manifold and converter, downstream O2 sensor wiring damaged due to coming in contact with exhaust pipe, or failed converter.


Check for any exhaust leaks and repair as necessary. Check wiring harness at O2 sensor and if damaged repair or replace. Keep in mind that catalytic converter have a 8yr/80,000 mile federal emissions warranty, so if you have less than 80,000 miles this will be covered undered the warranty and would need to be replaced by dealership.


We strongly recommend that if you need to replace the catalytic converter that you replace it with an OEM unit. Second choice would be a high-quality replacement part. There are many stories in our forums where folks replaced the cat with a cheaper aftermarket one only to have the code return shortly thereafter.



Hope this helps; also keep in mind that your feedback is important and I`ll appreciate your time and consideration if you leave some testimonial comment about this answer.

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Aug 24, 2012 | 2002 Honda Civic

1 Answer

2001 Toyota Camry - check engine light on - code p0420 - took to mechanic and he just hooked up code scanner and gave p0420 code - without checking anything else he said rear O2 sensor needs to be...


Hello: Code 420 normally indicates a catalytic converter is operating below 60% efficiency. You need to take your car to another shop and request a COMPLETE diagnosis not a dart board guess.

Sep 13, 2011 | 2001 Toyota Camry

1 Answer

05 Chevy Malibu P0420 code has set the check engine light.


Hi. P0420 is a catalyst efficiency code. 100% of the time you need a new catalytic converter. If you want to be sure it is the cat pull the O2 sensor before the cat and drive it, the sluggishness will go away because there is no more restriction.

Aug 26, 2010 | 2003 Chevrolet Malibu

1 Answer

Which is what I do to write the code P0420, as do I fix it? I change it?


So what is causing my P0420 code? There are a number of things that could be causing this p0420 catalyst efficiency code. The most common thing is the catalytic converter itself is no longer functioning properly. The other likely thing is the rear O2 (oxygen) sensor is no longer working properly. Other things could include exhaust leaks, damaged exhaust pipes, damaged O2 sensor wiring/connectors, plugged catalytic converter, etc.
OK, so I want to diagnose things, what do I do next? The easiest thing to do first is a visual check of things. Visually inspect the exhaust system for leaks, check the catalytic converter for dents, holes, severe discoloration, and check for a rattle inside. If any of those syptoms are there, the converter likely needs replacement. Then, visually inspect the downstream O2 sensor (behind the converter). Check for broken wires, obvious faults, etc. If all that checks out, you'll want to check the operation of the O2 sensor. To do that, you'll need access to a scan tool or oscilliscope. Check that the waveform is pretty steady. If the reading fluctuates then the sensor is likely bad and will need to be replaced.
On some vehicles such as some Subaru models an ECM reflash will fix the problem. As you can see a P0420 can be caused by many things, so if you have any doubts as to whether you can fix the problem yourself, please seek the advice of a professional technician. Good luck!

Answer found @ www.p0420.com

Jan 23, 2010 | 2002 Jaguar X-Type

1 Answer

1995 toyota tacoma 2.7 4cyl/5 manual/4x4 P0420 trouble code


sensors maybe aftermarket or bad there very sensitive may haft to replace!!!!

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3 Answers

Does PO0430 mean anything other than replace the catalytic conver


Yes the front O2 sensor(the one before the cat) could be lazy and cause this also.

May 25, 2009 | 1996 Mercury Sable

2 Answers

P0420 & P0430 codes 99 silverado


codes p0420 and p0430 are both post o2 sensors, the ones after the convertor, both sides if replacing

May 17, 2009 | 1999 Chevrolet Silverado 1500

1 Answer

Diagnostic codes


p1133 O2 sensor insufficent switching (pre converter, left bank)

p1153 ) 02 sensor insufficient switching preconverter right bank

P0420) catalyst system efficiency below threshold left bank

P0151) 02 sensor circuit high voltage pre converter right bank

and p1460 unfortunately I don't have handy..... But REALLY sounds like you need to replace the Oxygen sensors in front of the catalitic converters... There is dual exhaust coming from the engine to one muffler in that truck, there are a total of 2 converters and 4 O2 sensors (1 in front of converter and 1 behind it on both of the pipes) Front ones are definetely shot!!!! Be prepared to put the rear ones in shortly after!!!!! they are about $80 canadian each at the jobbers and probably more at the dealer!!!!!

Feb 24, 2009 | 1997 Chevrolet Tahoe

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