Question about Buick Park Avenue

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Had p0420 code on buick v-6. replaced cat. convrter, both o2 sensors and had tune up. light still comes on.

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  • Lee May 11, 2010

    Unless your in a state that checks for check engine lights to get them registered,why would you bother to change all of that stuff out for an emmission code that does not affect driveability..On a car thats probobly 10 years old?...There are also three o2 sensors on the 3800...one on each bank,and one behind the converter....if aftermarket convertor,I have seen them trip check engine lights before....I would put a piece of tape over light and drive it...lee.

  • Stephen Tucker
    Stephen Tucker Apr 19, 2013

    great answer ,,,,,,,,

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A code P0420 may mean that one or more of the following has happened:

  • Leaded fuel was used where unleaded was called for
  • An oxygen sensor is not reading (functioning) properly
  • The engine coolant temperature sensor is not working properly
  • Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
  • Retarded spark timing
  • The oxygen sensors in front and behind the converter are reporting too similar of readings.

Posted on Jul 24, 2009

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Posted on Jan 02, 2017

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2 Answers

Where is the O2sensor bank 1 at with a code of P0420. AND do I need to replace both sensors upstream and down. Or just the one that is showing up on the obd scanner.


Bank one is front side of cat. It should also tell u if it's cat 1 or cat 2. Depending on make an modle an yr. Catalyst 1 bank 2 is left cat back sensor. Catalyst 2 bank 2 is right cat back sensor. U do not have to replace upstream also. My o2 sensor Catalyst 2 bank 2 went bad. Had engine code for burning rich bank 2. Replaced tha down stream sensor an fixed problem runs better an better gas. This was in 97 gmc suburban. Idk what vehicle ur askin about. An I'm not to familiar with much about cars jus trucks an suvs.

Jan 02, 2017 | Cars & Trucks

4 Answers

I have 96 buick code p0420.I have replaced the converter and both 02 sensors.mil came back on code p0420. do not know what to do next, help.


fault code that is set when the Onboard Diagnostic II (OBD II) system sees a drop in converter efficiency. The OBD II system monitors catalyst efficiency by comparing the switching activity of the upstream and downstream oxygen sensors in the exhaust. The upstream O2 sensor in the exhaust manifold reflects the condition of the exhaust gases as they exit the engine. The downstream O2 sensor in or behind the catalytic converter reflects the condition of the exhaust after it passes through he converter.

Aug 02, 2008 | Buick Regal Cars & Trucks

2 Answers

I have a 2010 Buick Lacrosse that had the P0420 code. Replaced the Bank One Cat and the code and the check engine light came on again with the same code.


Did they use an original equipment cat from the dealer or an aftermarket one? Aftermarket cats can cause more problems than they fix. There very few that I will personally install.

are they monitoring downstream O2 sensor data to see if the O2 sensor may be biased high? If the cat is doing its job the downstream O2 sensors will read a lower voltage. Tricking the vehicle into running rich or lean will cause this voltage to change ( unplugging a coil will cause the Downstream O2 to read high as the cat tries to burn off the extra fuel. Unplugging a fuel injector will cause the downstream O2 sensor to read low as extra air is being pushed thought the cats). Making sure the downstream O2 sensors are capable of switching should be part of the diagnostics.

Are you sure they replace the right cat? There are 5 different engine options for this car. If you let me know what engine you have I can let you know what bank is bank 1.

Feb 20, 2015 | 2010 Buick LaCrosse CXL

1 Answer

Po 420 ?


code p0420 refers to cat converter system, bank 1 efficiency below threshold===causes --cat converter--wiring HO2S sensor 2

check that the cat converter is ok and replace the sensor. ( it is a HO2S sensor which will be different to a O2 sensor so get the right one )

Sep 12, 2014 | 1998 Chevrolet Astro

1 Answer

1998 buick century code p0420


code p 0420 refers to cat converter system bank 1---efficiency below threshold====causes ---cat converter---wiring---HO2S (heated oxygen sensor, the one in the exhaust manifold before the cat converter)
code p 0300 is random misfires
I would suggest changing the HO2S as it reports to the ECM on exhaust content so that the ECM can adjust the air/fuel ratio
so it is logical that if that unit is not reporting correctly, the cat converter may be having trouble burning off the gasses correctly
The oxygen sensor(O2S) reads the results of the cat converter operation and helps fine tune the ECM settings

Mar 10, 2017 | 1998 Buick Century

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

Code reading p0420 on '99 chevy s10 blazer, low catalyst threshold. Is this a bad catalytic converter or burned valves?


there are a couple of things to consider here. That code indicates that the O2 sensor behind the cat is not functioning like the engine computer expects-either the cat itself is not functioning properly or you may have an exhaust leak, or the heater circuit in the O2 sensor is faulty. The cat should get up to 600 degrees and it burns unburned fuel before ti gets to the 2ed O2 sensor-if it cannot do this, the CEL comes on and you have a P0420. Before replacing the cat, clear the code and see if it comes back-if it does, you might try first replacing the O2 sensor. If that doesn't work, get a 'oil fouler' spark plug adapter and install the O2 sensor in it, then install both into the exhaust-this 'shields' the tip of the O2 sensor and slows it down, fooling the engine computer into seeing it switching slowly, as it is supposed to do. Your only other choice is to replace the cat, and an aftermarket cat is much less than an OEM unit.

By the way, if you have burnt valves, a P0420 is the least of your worries.

Apr 01, 2011 | 1999 Chevrolet Blazer

2 Answers

PO420 code-service engine light is on


P0420 Catalyst System Efficiency Below Threshold (Bank 1) first thing to do is go back to basics do a tune up (plugs) check the plug wires spray them with water or windex if you get a miss change them, run some fuel injector cleaner through it, don't just throw parts on it ,that is'nt cheap

Mar 24, 2010 | 2002 Nissan Altima

1 Answer

Engine light codes 02-3151 and P0420


Code P0420 is a code for Catalyst efficiency. The computer is seeing the post Catalytic converter o2 sensor having too much activity. The Cat. should be cleaning the exaust of Hc, co, and Nox. The post cat o2 will read the o2 content in the exhaust and if it moves up and down too much or to high and low, the computer interperates that as the cat not ridding the exhaust of it's pollutants. This indicates that the converter is unable to preform properly. The problem could be a bad cat, o2 sensor (Pre- and/or post cat)or o2 sensor wires, or an air fuel mixture problem that has caused the cat to fail. Many other engine operating system failures can cause the converter to not work properly.

Sep 10, 2009 | 2000 Toyota Corolla

1 Answer

P0420 code


In case anyone else comes across this, there's a 90% chance the converter has failed. Check that the rear O2 sensor is working right using an oscilliscope or scan tool. If it's OK then the cat is likely needing replacement. More info at http://p0420.com

Jul 20, 2008 | Buick LeSabre Cars & Trucks

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