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Ron Z.
Ron Z., - Auto Tech -
Category: Mitsubishi
Satisfied Customers: 17949
Experience:  18+yrs. experience diagnostics & repair of all makes/models.
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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: 3 months ago.
Category: Mitsubishi
Expert:  Ron Z. replied 3 months ago.

Is this the 2.0L or the 2.4L engine?

Customer: replied 3 months ago.
2.0
Expert:  Ron Z. replied 3 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:https://goo.gl/wZ2Ryc

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Expert:  Ron Z. replied 3 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 3 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 3 months ago.

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