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I get OBD code 421. I have repalced everything except teh Cat Converter. The Eclipse has two converters. I am not sure which one to replace?

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SOURCE: catylitic converter

without a doubt, a plugged converter or your engine is still running rich. Any CEL Codes?

Posted on Sep 14, 2008

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SOURCE: check engine light

did you ask the dealer if the car an an emissions recall for this problem

Posted on Nov 13, 2008

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SOURCE: Cat-Converter-Is it necessary to make car run good?

you only need the converter if you live in an emissions state and have to pass inspections. Won't pass without it

Posted on May 05, 2009

  • 466 Answers

SOURCE: getting po104 reading on yhe


Posted on Dec 19, 2010

  • 2019 Answers

SOURCE: A recent scan indicated the

These are all transient symptoms, so I would not panic.
What I would suggest is putting in a tank of premium and fuel dryer.
A little water or cheap low octane gas can cause these symptoms.

If it happens again, a second try would be plug wires, because crossover in the wires will also do it.
Reducing plug gap will also make problems like this less noticeable.
You can go as low as .028".

Start with the cheap tries first, then worry about the more expensive things.

Posted on Jan 21, 2011

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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

1 Answer

Evap and cat converter monitor not showing ready when trying to smog 2001 daewoo nubira

you hav'nt mentioned the car, but i'd get the OBD codes read as will show which sensor/switch is at fault

Jul 04, 2014 | Daewoo Nubira Cars & Trucks

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

My check engine light is on and has a 421 error code? What does this mean?

OBD Code P0421refers to
Basically this means that the oxygen sensor downstream of the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.
A code P0421 may mean that one or more of the following has happened: The catalytic converter is no longer functioning properly An oxygen sensor is not reading (functioning) properly A sparkplug is fouled up

Jul 21, 2011 | 2006 Porsche Cayenne

1 Answer

O2 sensors keep failing and a evap leak code i put new gas cap did not cgehan

Are you sure it's 02 sensors? Just because code says 02 sensors doesn't necessarily mean they're bad. Bad 02 sensor code can also indicate bad cat. converter. If 02 sensors read exact same reading before and after converter they will trigger OBC to "sensor fault" - they should be reading 2 different readings. (Pre CAT/POST cat) I'd suggest replacing cat converter before replacing sensors.

Mar 14, 2011 | 1999 Toyota Camry Solara

3 Answers

Po420 code monte carlo ls 2001

PO420 code is for catalyst system efficiency below threshold (bank 1) Usually it means that it is time for a new catalytic converter on bank number 1. Cylinder bank number 1 is always the bank with the number 1 spark plug in it.

Apr 20, 2010 | 2001 Chevrolet Monte Carlo

1 Answer

2000 E-350 V-10. P0174 with exhaust fix

Before I even finished reading this i knew you had a clogged cat. If you have fixed everything you mentioned i would lean towards a clogged cat. This is a large reason this code pops up.

To check this you could pull that cat out and drive around and see if the code comes back, make sure to stay away from the police though.


Mar 03, 2010 | 2000 Ford Econoline

2 Answers

Engine code p0420

The P0420 code is set when the oxygen sensor after the catalytic converter gives a reading the same or very close to the reading of the oxygen sensor(s) before the catalytic converter.

There are usually three possible causes...
The front oxygen sensor (before the cat) is giving an incorrect reading.
The rear oxygen sensor (after the cat) is giving an incorrect reading.
The catalytic converter is not working properly.

Since you recently replaced the cat, the logic points to the sensors, but it could be the cat again. Aftermarket converters are usually smaller than OEM and don't last as long.

For a more thorough description of the code and troubleshooting visit

Our Subaru encountered this issue several years ago under warranty. The first time the dealer replaced the front oxygen sensor. This was the cheapest repair and since it was an intermittent problem I am sure they hoped to delay the issue until the warranty was up. The second time they replaced the catalytic converter and we haven't had any more issues.

Jan 08, 2010 | 2000 Honda Civic

1 Answer

What does Code 2097 for a 2004 2.5 Subaru and how do I repair


Nov 27, 2009 | 2004 Subaru Impreza

1 Answer

01 eclipse

P0 421= Catalytic Converter(s) Below normal flow
P0 455= Evap system leak .. Gas Cap or Carbon cannister or possibly evap lines
P0 123=Drive cycle Incomplete ( no Biggy )
P0506= Idle air control motor..
Doesnt sound like that big of a deal. at our shop.., we'd replace the converter(s) Check the gas Cap for the evap code.. And replace the Idle control motor. The code P0123 should be cleared By ODB2 at that point and should be good to go.. Lemmie know.. Thanks

Mar 11, 2009 | 2001 Mitsubishi Eclipse

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