What does DTC code P0431 on Mitsubishi montero mean and is this an emergency?

Answer 1

Trouble code P0431 means: Warmup Catalyst Efficiency Low (Bank 2)

This means that your driver side 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 so even if it were bank 1 you would still be replacing them both anyway. 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/