[jrharley97]

I have a cod p0421 & p0431 can anyone help me I chang all 4 o.s

[rsmenterprises]

You probably need to change your Y pipe with the 2 catalytic converters on it. That error refers to both sides of the engine, both converters. When they clog it throws that error. People commonly point the finger at the O2 sensors but the fact that they are reporting the code usually tells us that they are working fine and that the issue is with the emissions and exhaust not being up to standard. This is my response from another site with the same question:

This means that your... catalytic converter is not running is it's most efficient state. You will probably need to replace the Y pipe that runs from each side of the motor into the single pipe. It has two catalytic converters on it... Don't confuse the Y pipe for the OTHER catalytic (single) converter that is located downstream on the single pipe. The Y pipes run $250-$450 for the part on the internet and if you don't know how to change it you will need to pay a muffler shop to put it on for you. Alternately it may be caused by bad gas, a faulty O2 sensor, or various other things but the fact that the sensor is reporting the error usually means the cats are going out and the sensor is fine. It happens around 100k+ with the Monteros. You can try pulling the battery to reset the code and try using premium gas from now on and see if it returns but it probably will. Hope that helps.

Here is an example of what it looks like:
http://www.drivewire.com/vehicle/2001-mitsubishi-montero/catalytic-converter/

[rsmenterprises]

Also:
* The catalytic converter is no longer functioning properly
* An oxygen sensor is not reading (functioning) properly
* A sparkplug is fouled up