I just got my 2004 Mitsubishi Endeavor Limited 4WD back after 4 weeks in the shop, there was a problem with the transmission and $1800 later the transmission was completely replaced.

I was driving when the Service Engine Soon light reared its ugly face. Luckily I recently bought a OBD-II Diagnostic Transceiver so I was able to quickly find any problem there may be. The software returned a single error code P0431 which after a Google search I found to be linked to Warm Up Catalyst Efficiency Below Threshold Bank 2.

I must preface the actual question by saying I have no intention to do the work myself.

  • I would like to know exactly what this error means.
  • Might this problem be related to the service that was just completed?
  • How long can I go without fixing this before it becomes a serious problem?
  • How much might it cost me when it comes time to do the repairs?
  • Someone mentioned to me this problem would affect my car's fuel efficiency. Is this true?

3 Answers 3



How many miles? If over 100K or close to that, your catalytic converters could be bad. when they go bad, they can block your exhaust flow which will effect power performance and fuel efficiancy. cats work by a material that glows red hot when running. unburnt fuel is flashed off when going through them and does not get to atmosphere. the material that is the catalytic material will start breaking down, melting and clogging up the flow. get them replaced. look forward to another $1K++ bill. the cats themselves are $200 to $400. You may have two of them in your system. then you have labor for tearing your exhaust system apart and putting it back together.

  • 2
    Try replacing the oxygen sensor before replacing the catalytic converter, generally they are cheaper, and they often go bad before the cat itself does.
    – mac
    Jul 16, 2012 at 16:58
  • Thanks, I have recently sold the car. The dealer said they'd replace the catalytic converter as part of the deal. Oct 18, 2013 at 17:38

I had the same thing on my 2001 full size Montero with 183K miles. It would not pass CA smog test. I tried running 2 bottles of Cataclean but that did not work. I bought the OEM cats online for 800 bucks including shipping. Paid a local muffler shop 300 to install. It worked.enter image description here


After cat O2 sensor code 421 and 431 cause no damage to vehicle.

  • Cheap fix, buy 18mm spark plug anti-foulers

  • Drill out center with 1/2 drill bit.

  • Screw screw fouler into exaust.

  • Screw O2 sensor into fouler.

Cost $4

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