Question about Mazda MX-3

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I have a 2004 Mazda 3. Check engine light came on. After a couple days it went off. Then a couple days later it came back on. Code was P0421, Warm up catalyst effeciency below threshold bank 1. I am figuring it is either oxygen sensor or catalytic converter. How can I diagnose this?

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  • stephen_kats Dec 03, 2008

    I have a similar problem, I'm guessing. Last winter, and now again this winter I've had problems with my Tribute idling and while driving. Even at idle, the engine runs very roughly and has the same symptoms while driving. I've been told that it is possibly the O2 sensor, but want to be sure before replacing it. Also, I believe there are 4 in my vehicle and am not sure which one(s) to replace. I haven't noticed any decrease in fuel efficiency so I'm guessing it wouldn't be a primary sensor?

  • Anonymous Mar 30, 2014

    Catalyst converter sensor

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It can be hard to say which it is, because either one will trip the the code, but in this case I'd place my bet on it being the O2 sensor because it says "warm up" - the O2 sensor has to be up to temperature (around 900 degrees) to function. The computer watches for the O2 sensor to heat up in a certain amount of time, and if it doesn't get to temperature soon enough, the computer diagnoses a faulty heater for the O2 sensor and trips the code.

For the record, this would be the secondary O2 sensor, the one after the cat. That's the one that is used to determine cat functionality and efficiency. The primary sensor, before the cat, is used to fine-tune the fuel delivery.

Posted on Aug 08, 2008

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2002 mazda mpv sputtering


Error Code: P0421
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CODE P0421. DEFINITION-CATALYST EFFICIENCY LOW-BANK 1. EXPLANATION-THE OXYGEN SENSORS MONITOR THE CATALYTIC CONVERTERS ABILITY TO STORE OXYGEN. PROBLEM CAUSES- 1. CATALYTIC CONVERTER DEFECTIVE (FAILURE POSSIBLY DUE TO #2,3, OR 4) 2. ENGINE MISFIRE OR RUNNING CONDITION. 3. LARGE VACCUM LEAK 4. ENGINE OIL LEAKAGE INTO EXHAUST-VALVE GUIDE SEALS, PISTON RINGS. THE PCM HAS DETERMINED THAT THE WARM UP CATALYST EFFICIENCY FOR BANK 1 IS BELOW TRESHOLD FOR THE CURRENT ENGINE OPERATING CONDITION. (BANK 1 IDENTIFIES THE LOCATION OF CYLINDER #1, WHILE BANK 2 IDENTIFIES THE CYLINDERS ON THE OPPOSITE BANK).

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Code - P0421 text as follows:

WARM UP CATALYST BELOW THRESHOLD - BANK 1
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First pls check if ur car needs a tune up? if not done since long (3yrs/60K miles), do the tune up and see if it helps. or else u will need to do as explained above.

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I have a 1996 Mazda 626, the check engine light came on and the car has been redlining the the temp gauge. I got the engine codes which are: p0117 and p0421. Does anyone know how to solve this problem?...


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