How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Ron Z. Your Own Question
Ron Z.
Ron Z., - Auto Tech -
Category: Mitsubishi
Satisfied Customers: 18791
Experience:  18+yrs. experience diagnostics & repair of all makes/models.
Type Your Mitsubishi Question Here...
Ron Z. is online now
A new question is answered every 9 seconds

My smog test failed. P0421- catalyst 1 efficiency below

Customer Question

my smog test failed. P0421- catalyst 1 efficiency below threshold bank 1.
JA: What specifically didn't pass the test? When was your last oil change?
Customer: oil change was done last Monday. obdII failed, obd bulb check failed.
JA: What is the model/year of your Mitsubishi?
Customer: 2006 lancer
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: yes. have not tried anything as yet. thinking about changing spark plugs and o2 sensor.
JA: Anything else you think the mechanic should know?
Customer: no
Submitted: 11 months ago.
Category: Mitsubishi
Expert:  Ron Z. replied 11 months ago.

Is this the 2.0L or the 2.4L engine?

Customer: replied 11 months ago.
Expert:  Ron Z. replied 11 months ago.

Great, thanks. The P0421 is a fairly straight forward code pointing to a failing warm-up converter. The Oxygen Sensors measure the amount of emissions in the exhaust going into, and then out of the converter. When they see that the converter is not "cleaning" the exhaust enough to meet specs, the Converter is deemed "below threshold" and the P0421 is set. Make sure there are no current cylinder misfires. Make sure there are no leaks in the exhaust system. Which, if there were, you should be able to hear. Sounding like a repeating "tick" or "puff" from the exhaust. Usually more predominate when the engine is cold. Make sure to include the exhaust manifold when checking for leaks/cracks. If these check ok, then most commonly, the Converter is "on it's way out" and will need replacement in the near future. Keep in mind, you will need a replacement converter that meets specs. Those "universal" or "discount" converters sold at some after market parts stores do not meet specs, and will not cure a P0421. On this vehicle, the "correct" warm-up converter is built into and part of the exhaust manifold. For an image location view, click here:

If you found this answer helpful, please help me by leaving a rating using the "stars" at the top of the page or click 'accept'.
Rating or accepting my answer does not cost you extra. It allows me to be credited for answering your question. Please do the right thing and rate or accept!

Expert:  Ron Z. replied 11 months ago.

Were there any other questions you had on this? Any other information I can provide to better assist on this?

Expert:  Ron Z. replied 11 months ago.

Hi! I noticed this thread was still open and you did not leave a rating , so I wanted to check in and see if there was anything else I could do for you on this issue? Did you have any other questions? Was there something I missed that you wanted addressed? If you have no further questions and simply forgot to leave a rating, please take a second and use the stars at the top of the page to rate our Q&A session. Thank-you!

Expert:  Ron Z. replied 11 months ago.


Related Mitsubishi Questions