Question about 2003 Audi A6

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Codes p0421 and p0431

I am told these codes indicate cats need to be replaced. I find it hard to believe both would go bad at same time. Any advice?

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Check for air leak on vacuum hoses, and exhaust plus test health of O2 sensors b4 doing the CAT b/c CAT are the most expensive.

Posted on Oct 10, 2010


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2003 ES. What are the probable causes of DTCs P0421 and P0431?

code p 0421 refers to warm up cat converter bank 1--efficiency below threshold ===causes ---cat converter---wiring---HO2S 2
( heated oxygen sensor)
code p 0431 refers to warm up cat conveter bank 2--efficiency below threshold === causes cat converter --- wiring---HO2S 2
as the same HO2S appears in the cause of the fault I would be inclined to change it first
the HO2sS sensor is the one in the exhaust manifold or before the cat converter
and 2 is in the front bank of a V engine
use the special socket when removing/ replacing these sensors as an incorrect tool is the main reason for wire failures on the sensors

Dec 17, 2015 | 2003 Mazda MPV

1 Answer

Lots of codes now car wont start

The codes would appear to be an Oxygen Sensor
problems & Failed Converters,smell or not

Can't smell after all the expensive rare metals have burned off
from over heating for years

Need to find out why they failed

Check for cylinder misfires & fuel trim issues

Mar 27, 2014 | 2001 Audi A4

1 Answer

2007 Mitsuibishi Eclipse GT

If this is a V6 model, Bank 1 would indicate one side of the motor. Also, if it's the V6 model, I believe there are 3 O2 sensors, one per Bank, and one downstream(post-cat). If it is the V6 model, and it indeed is Bank 1, then just replace the O2 sensor, that's the most likely cause.

Jan 04, 2018 | 2007 Mitsubishi Eclipse GT

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P0421 code bank 1 warm-up efficiency below limit

P0421 code is that the cat. converter is below efficiency. 99 per cent o the time the cat. needs to be changed. but if you have a scan tool watch both bank 1 O2 sensor 1 and bank 1 sensor 2.
cold they should be almost the smae but when its at operating temp, and running down the road at crusie speed the frt o2 should switch rapidly between .150 mv. and .850 mv. and the rear o2 should read about .600 mv. if they read the same the cat is ng and will have to be replaced.

Oct 28, 2013 | 2001 Mazda Millenia

1 Answer

Stumble @ 30-40 MPH under ligh acceleration

Problem sometimes is with cheap parts they are bad.Good wires,good plugs,good coils.

May 05, 2013 | 2004 Mitsubishi Endeavor

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Code P0431, code P0421

I experienced the same. Rather than replace costly cats I tried to change gas to highest octane - and voila! - all lamps get off.

Aug 29, 2017 | 2007 Mitsubishi Eclipse

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2001 mitsubishi galant with a P0421 code whut to do

this code relates to catalytic converter efficiency, could be a bad o2 sensor, bad cat , prob gonna need a mechanic to diagnose, if you do decide to replace o2 sensor you want to change the one behind cat, tghis one monitors cat function

May 18, 2010 | 2001 Mitsubishi Galant

2 Answers

What are codes P1492, P0421 & P0431 on the 2002 Mazda Millenia?

po421 warm up catalyst efficiency below threshhold bank 1, 431 same on bank 2..

Feb 08, 2010 | 2002 Mazda Millenia

1 Answer

What are code p0342 and p0431

342 is camshaft position sensor and 431 is your catalytic converter may be getting weak. If this was my car I would do the 342 code and see if the other code for the cat comes back. Tell the shop fix the camshaft sensor. If the sensor is bad your car can still run but its possible you could set a cat code because
of it but on the other hand you cannot set a camshaft code because of a bad cat.

I am a smog tech in California

Aug 10, 2009 | 2005 Chevrolet Aveo

1 Answer

P0421 code on recently purchased(from dealer) 2002 RS


So you have a P0421.

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

As far as running conditions if it performs normally, you most likely need a catalytic converter. The good part of this is there shouldn't be any abnormal running conditions, so it should be fine unless the engine is running very rich for some reason. (it will usually smell badly when this happens) However, on rare occasions the cat can get hot and melt down causing an exhaust restriction..bad gas mileage, low power, and possible fire potential, but most likely this is not the case. (this is generally speaking here and covering all the bases... )

The most common thing with P0421 that the fuel injection computer detects that the cat is not performing to federal specs an need to be replaced. You will need to have it fixed before your vehicle inspection.
Good luck and I hoped this helped you.

Dec 27, 2008 | Mitsubishi Eclipse Cars & Trucks

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