Code P0431, code P0421

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

The code 421 is a problem with the catalytic converter or O2 sensors on bank one. Under normal conditions the front and rear sensors should be giving different readings back to the computer and they are more or less the same.
Code 431 is the same thing for bank two.
Unless you find a big exhaust leak, I doubt this is something you can fix yourself. You would need a scanner to watch the readings coming from the O2 sensors and the engine temp among other things to be able to determine the cause or causes.

Converter is not working as efficiently in Bank 1 and Bank 2 (two codes)...

P0421 - Warm Up Catalyst Efficiency Below Threshold (Bank 1)
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.

Symptoms: You will likely not notice any drivability problems, although there may be symptoms. The code may be more likely to occur after the engine has had numerous cold starts in the past 1-2 days.

Causes: 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

Possible Solutions:
Measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

P0431 - Warm Up Catalyst Efficiency Below Threshold (Bank 2)
Basically this means that the oxygen sensor downstream of the catalytic converter on bank 2 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.

Symptoms: You will likely not notice any drivability problems, although there may be symptoms such as a rough/hard idle when cold.

Causes: A code P0431 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
There is an exhaust leak

Possible Solutions:
Inspect for exhaust leaks. Next step is to measure the voltage at the oxygen sensor on Bank 2. In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.


Hope this helps.

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

taken from: http://forums.evolutionm.net/lancer-troubleshooting/349095-code-p0421-call-mitsu-before-you-do-anything.html

Code P0421- Call Mitsu before you do anything! Icalled my nearest dealership today and for those who dont know, if yourcheck engine light is on and its showing fault code P0421 this may befrom a factory recall on the warm-up converter and a PCM reflash.MILEAGE OF THE CAR DOES NOT EFFECT THIS RECALL. Anyone whos had problemwith a dealership not doing the repairs because of mileage needs tocontact another dealer or the owner of the dealership. This is what theservice manager told me today so though I'd share. Getting mine fixedtomorrow at 10am, I cant wait.

For more information about this code Google "Fault Code P0421" You'll get tons of information Last edited by Swankzer; Jun 2, 2008 at 12:31 PM.

taken from: http://forums.evolutionm.net/lancer-troubleshooting/349095-code-p0421-call-mitsu-before-you-do-anything.html

Code P0421- Call Mitsu before you do anything! I called my nearest dealership today and for those who dont know, if your check engine light is on and its showing fault code P0421 this may be from a factory recall on the warm-up converter and a PCM reflash. MILEAGE OF THE CAR DOES NOT EFFECT THIS RECALL. Anyone whos had problem with a dealership not doing the repairs because of mileage needs to contact another dealer or the owner of the dealership. This is what the service manager told me today so though I'd share. Getting mine fixed tomorrow at 10am, I cant wait.

For more information about this code Google "Fault Code P0421" You'll get tons of information Last edited by Swankzer; Jun 2, 2008 at 12:31 PM.

That code is for the 02 sensor down by the catalytic converter, that one should be changed.

Hi,

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.

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.