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Thread: Comments on CEL P0420 (bank 1 o2 sensor)

  1. #1
    Senior Member HondaSUV Mentor
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    Default Comments on CEL P0420 (bank 1 o2 sensor)

    my 1999 crv has 162k miles on it. i replaced the bank 1 (front) oxygen sensor in october 2007 at 150k miles with a denso brand. a couple days ago the CEL came on. i checked it with a code reader. the only code was P0420. is it really possible that the denso sensor needs replacement after 12k miles?
    over that time the crv has been used almost exclusively as a local driver.

  2. #2
    FFNC Site AdminHondaSUV Elder Racoon's Avatar
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    Actually, I think the P0420 means your catalytic converter is bad, not that the O2 sensor is having problems.

    Do you track your fuel mileage? Has your MPG decreased recently?

    The Racoon

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    i only drive in-town, so i don't bother keeping track of mpg or cost.

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    my records indicate that i replaced the pre-cat o2 sensor at 150k miles. post-cat sensor probably original. if that sensor is failing, would it not read that the cat is not doing its job properly?

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    FFNC Site AdminHondaSUV Elder Racoon's Avatar
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    http://www.aa1car.com/library/p0420_dtc.htm

    If the downstream O2 sensor is bad (heater circuit not working, loose or corroded wiring connector, contaminated sensor element, etc.), the OBD II system should detect the fault and set an oxygen sensor code. The same goes for a bad upstream O2 sensor. In either case, the presence of an O2 sensor code should prevent the catalyst monitor from running and setting a false P0420 code.
    So no, I don't think a failed secondary/post-cat O2 sensor will set a P0420.
    I believe a failed secondary/post-cat O2 sensor throws a P0141.

    But I could be wrong, and hopefully someone else will take time to comment.

    Good luck.
    The Racoon

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    Senior Member HondaSUV Mentor mezz64's Avatar
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    Racoon is correct, a P0420 code does not indicate a failing downstream O2 sensor, but rather a failing cat. The code description is something along the lines of "catalyst inefficiency".

    You will also get this code if you have an exhaust leak at the flanges between the cat and b-pipe exhaust sections, ask me how I know
    John
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    Senior Member HondaSUV Elder 'Curly Q Links''s Avatar
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    P0420 (in the Honda manual) is saying that the computer thinks the cat isn't working, because there's little difference comparing the (signal from the) secondary sensor to the (signal from the) primary sensor.

    There's a different code for a failed heater element.

    The best test might be a laser thermometer.

    DTC P0420 Catalyst System Efficiency Below Threshold

    Catalytic Converter Replacement

    Last edited by 'Curly Q Links'; 08-11-2011 at 10:31 PM. Reason: clarity
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    Senior Member HondaSUV Mentor mtxjohn's Avatar
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    12,000 miles in 4 years? That would be considered severe for the maint schedule. Could the short trips (loading the cat with fuel) and never getting to proper operating temp caused this failure?

    97 V-Heated leather!! 126hp (270K mi) sold
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    i cleared the code, but it came back. everything is short distances for me, can't be helped.

    before the light originally came on, i had been noticing a "fluttering" sound when accelerating harder. it did sound like some kind of exhaust leak. it doesn't seem to be noticeable like a typical leak. at some point i will try and identify the noise. hopefully it is the cause, and not the cat.

  10. #10
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    cleared the code again using a scan tool. cel returned within 30 miles.
    not sure if i have a air leak in the exhaust system; checked but couldn't feel anything. noticed some movement in the front of the crankcase with the fans not running, but could be caused by the belts.

    again, i believe the rear o2 sensor is original w/162k and 12 yrs on it. could it be failing and causing this issue?

    cat looks solid, no noticeable issues.
    Last edited by rwh63; 08-11-2011 at 10:38 AM.

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