P0420 and bad oxygen sensors

Discussion in 'General Motoring' started by johngdole, Mar 16, 2009.

  1. johngdole

    johngdole Guest

    That's why owners shouldn't replace catalytic converters without
    checking sensors first:

    Feb 2009 Motor.com article:
    johngdole, Mar 16, 2009
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  2. johngdole

    clare Guest

    Helps to have a 4 gas analyzer to check the exhaust pre and post cat -
    then you KNOW if the cat is working.

    Also having a good scanner that can "track" the O2 sensor helps - you
    can see what the sensor is doing. Often switching sensors from pre to
    post will also tell you if the sensor is the problem. If moving the
    sensors still gives the same code, without an o2 sensor code, it id
    VERY likely the cat IS week.
    clare, Mar 16, 2009
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  3. A catalytic converter is VERY robust, and unless you feed the car leaded
    gasoline or some other unauthorized substance, the CAT ought to last the
    life of the car.

    The only CAT that I have ever replaced was physically damaged by reason of
    losing a battle with a very large hammer. I had an '89 Ford motorhome
    (motorhome on a Ford chassis) that I thought had a CAT problem in about
    2003. The CAT was fine. The muffler was the problem. The muffler was a
    significantly less costly repair. The motorhome is still going strong with
    the same CAT that I thought needed replacing back in '03.

    I'd not expect any '96 or newer car or truck sold in the USA to have a CAT
    problem for several more years.

    The sensors, on the other hand, are not nearly so robust and the environment
    they live in is very hostile. I'd suspect a sensor failure long before a CAT
    Jeff Strickland, Mar 17, 2009
  4. johngdole

    Ray O Guest

    Pre OBD II vehicles generally did not have a post-cat O2 sensor so there was
    no on-board way for the vehicle's diagnostic system to evaluate the
    performance of the cat so you would not get a check engine light related to
    cat performance. The only way to evaluate cat performance in a pre-OBD II
    vehicle is with an exhaust gas analyzer (also the best way to evaluate cat
    performance in an OBD II vehicle).

    The catalyst in a catalytic converter does not get used up or wear out so it
    could theoretically last forever. What happens when cats fail is that the
    catalyst becomes coated with contaminants like particulate (soot) so that
    the exhaust gas passing over the catalyst bed does not come in contact with
    the catalyst so no catalytic action occurs, or the converter develops a hole
    that allows exhaust gas to escape and cool air to enter, or interior
    corrosion allows the bed holding the catalyst to collapse or come apart,
    clogging the flow of exhaust.

    Your motor home probably saw a lot of highway miles, with a nice hot exhaust
    that minimized buildup of contaminants on the catalyst, prolonging its life.
    Ray O, Mar 18, 2009
  5. And, to clear up an important detail, I thought my CAT was clogged. My fuel
    mileage went into the dumper and the power fell off even more. I had every
    indication of a motor that could not breathe, and I suspected the CAT as the
    problem. It turn out the muffler has dozens of baffles and stuff inside, and
    something came apart and blocked the passages that the exhaust goes through.

    The CAT that failed the encounter with a hammer was having a connection
    problem, the clamps would not hold because there was not enough bite, I
    sought to enlarge the hole so the pipes would fit together deeper. The plan
    was a good one, the execution was bad. Oh well ...

    So, my position is that the CAT is very robust, and not a likely failure
    item. If the sensor reports a failure of the CAT, my money is on the sensor
    first, and the CAT second.
    Jeff Strickland, Mar 18, 2009
  6. johngdole

    Scott Dorsey Guest

    Okay, we have a problem here. There are basically two kinds of converter
    problems. First of all we have problems that actually block the exhaust
    line, which are the kind you're talking about. And those problems, as you
    note, are pretty rare.

    But there are other converter failures, where the converter no longer is
    effective at reducing emissions. If you have an older car and you live in
    a state that doesn't do emissions testing, you'll never know when these
    failures happen. And sadly, they happen all the time.

    If you have a newer car with a second O2 sensor on the output of the
    converter, and the converter fails, you'll get an error message saying
    that the converter has failed. And if you are in a state with emissions
    testing, you'll be told you didn't pass the test.

    You'll find if you look that the average lifespan of a converter is somewhat
    short of 100,000 miles, but that they mostly fail in such a way that you
    aren't noticing.
    Scott Dorsey, Mar 18, 2009
  7. johngdole

    Ray O Guest

    Yup, the effect is like Eddie Murphy sticking a bannana up the tailpipe in
    Beverly Hills Cop.
    I don't think that cat failure is as rare as you think; it is just that cat
    failure is not detected very often, which is why the OBD II standard
    included a way to check cat efficiency.
    Ray O, Mar 18, 2009
  8. johngdole

    clare Guest

    You'd be surprised how marginal many cats are from new - the calaysts
    are expensive so as little as possible active material is used.
    Anything goes wrong that reduces the effectiveness of the cat AT ALL
    and you fail your e-test.

    Coolant leak burning antifreeze can poison the cat. The O2 sensor goes
    bad, causing the engine to run rich - dead cat. Burning oil? Dead cat.
    Leaded gas? dead cat.

    Lots of reasons for cats to fail - and many do. The thing to remember
    is you need to solve the problem that killed the cat - as they
    GENERALLY do not die on their own. MANY times a bad O2 sensor will
    indicate a bad cat - and sometimes it will CAUSE a bad cat.
    clare, Mar 18, 2009
  9. johngdole

    me Guest

    So how can one remove it permanent and
    still have the car computer control car

    I have 2000 Mazda Protege ES with sensor
    before and after cat
    me, Mar 18, 2009
  10. johngdole

    Ray O Guest

    What do you want to remove permanently?
    Ray O, Mar 18, 2009

  11. It is my humble opinion that this type of problem -- loss of
    effectiveness -- is rare. I would suggest that a CAT should last the life of
    the car for the vast majority of owners. Yes, the CAT might fail, but I
    suggest that the first thing to fail will be the sensor.

    The sensor is a relatively fragile device that lives in a hostile
    environment. The sensor will fail before the CAT.

    I've not heard of a lifespan for a CAT. I drove a '94 BMW until it read
    225,000 miles and was hit by another driver. I have a current '94 BMW with
    over 130,000 miles and the CAT is strong. My daughter has an '00 BMW that is
    quickly approaching 150,000 miles and the CAT works good. I have a '97 F150
    with 110,000 miles and no problems with the CAT. Every car I own has more
    miles than you suggested as the life of the CAT. Nobody in my family has
    ever bought a CAT, and the only CAT I've bought was to replace the one that
    I badly deformed while trying to correct an unrelated exhaust system

    I'm not sure there is a stated life for a CAT, and I seem to recall reading
    that they want the CAT to be a life item for the car that it is installed
    in. As far as I know, the only thing that can happen, short of physical
    damage -- the seams opening, that sort of thing -- is that the operator can
    feed the vehicle's fuel supply some kind of product that is not good. I
    don't know what not to feed a CAT, other than leaded gasoline, which I don't
    even know where one could buy that anymore. I suppose there's a possibility
    that aviation gas might be leaded -- but I don't know one way or the
    other -- and it could be put into a car.

    My advice is that if one gets a Failed CAT Code from the OBDII system, test
    and replace the sensor before suspecting the CAT itself as a failure item.

    The OBD II system has a sensor that looks specifically at the CAT, but the
    actual failure item is the sensor itself, and the OBD II system has not got
    a means of checking and evaluating the sensors themselves. Any code that
    involves a sensor in the exhaust stream is likely to be caused by the sensor
    itself, not the hardware that the sensor is looking at.
    Jeff Strickland, Mar 18, 2009
  12. johngdole

    Scott Dorsey Guest

    Could be, but how would you know?
    How do you know any of the converters are good? Are you in a place where
    you are getting annual emissions testing?
    I'm sure this is the case, but that's not to say you weren't driving the
    car with a completely ineffective converter for hundreds of thousands of
    Scott Dorsey, Mar 18, 2009
  13. johngdole

    clare Guest

    Remove what?
    On a 2000 you need both sensors and the cat.
    clare, Mar 18, 2009
  14. Check the sensor.

    Except I would fail smog check long before I drove thousands of miles, not
    to mention tens, or hundreds of thousands.
    Jeff Strickland, Mar 18, 2009
  15. johngdole

    clare Guest

    I would suggest that you don;t know what you are talking about. The
    sensor that monitors the catalyst is just a second o2 sensor. If the
    second sensor tracks the first sensor, the converter is not
    functioning. It's that simple. It is possible, but not likely, that
    the conveerter could fail in such a way that the sensor would not pick
    up the failure, but extremely unlikely that the sensor would indicate
    a bad cat if it was functioning properly. Both sensors would need to
    fail, and in such a way that they did not cause an O2 sensor failure
    code, in order for this to occurr. EXTREMELY unlikely, when you
    actually know how they work.

    When I say it is POSSIBLE the cat could fail and not be caught by the
    sensors, it is because the oxidation catalyst uses up oxygen, and the
    reduction catalyst frees up oxygen, but generally not the same amount
    each way. The front O2 sensor oscilates with the O2 concentration of
    the exhaust gas. When the exhaust is lean, the oxidation catalyst
    works making co into co2 and burning off unburned hydrocarbons.- so
    the second sensor does not track. When the exhaust is rich the
    reduction catalyst works,removing O2 from NOX, making nitrogen and O2.
    The O2 is generally stored by the cat to feed the oxidation catalyst,
    so the O2 concentration of the exhaust remains more or less constant
    at the back sensor. Multiple mode failures in the converter could,
    concievably, occur which would provide a relatively constant O2 level
    at the rear sensor without fully treating the HC, CO and NOX.
    clare, Mar 18, 2009

  16. I know what the sensors monitor.

    What I said was that there is no sensor to monitor the sensors. The sensors
    send false data, particularly in the case of the O2 sensors _because_ the
    environment that these sensors operate in is very hostile. Given the
    specific code being discussed, P0420, all I'm saying is that the sensor is
    more likely a failure item than the CAT.

    I'm not looking at a CAT failure that is not reported, I'm looking at a
    reported CAT failure that does not really exist.

    We HAVE a code. It specifically points to the after-CAT sensor because the
    code that is generated can never happen from an upstream sensor.

    We have a false report of an very costly item failing. That's my story, and
    I'm sticking to it.
    Jeff Strickland, Mar 18, 2009
  17. johngdole

    me Guest

    remove the cat
    me, Mar 19, 2009
  18. johngdole

    Ray O Guest

    Why do you want to remove the cat?
    Ray O, Mar 19, 2009
  19. johngdole

    Ray O Guest

    We've been through this before.

    Your premise that the code that is generated can never happen from an
    upstream sensor is incorrect. P0420 is generated IF the signal from sensor
    #2 (post-cat) looks like the signal from sensor #1 (upstream sensor). When
    the system is warmed up, the signal from S1 oscillates, and on an
    oscilloscope, looks something like a square wave because the oxygen content
    in the exhaust gas is oscillating. After the exhaust passes through the
    cat, the O2 content in the gas is evened out so that the signal that S2 puts
    out has less variance in amplitude (height of the wave) and frequency
    (number of waves per minute) and so the signal from S2 looks different from
    the signal from S1.

    If the cat is bad, or if you took the cat out of the system and replaced it
    with a piece of pipe, the O2 content doesn't get evened out and so the
    signal from S2 looks like the signal from S1. IOW, the signal from S2 looks
    dead if the cat is working and looks lively when the cat is not working. If
    the ECU sees a lively signal, that means the cat isn't working and it sets

    If you get P0420, it is a good idea to check the sensors because it is easy
    to check, they generally have a shorter life than a cat, a sensor is cheaper
    than a cat, and a bad S1 can cause premature cat failure, but the cat should
    also be changed because the likelihood of a false P0420 is almost nil.
    Ray O, Mar 19, 2009
  20. johngdole

    me Guest

    Cause I don't have the money to replace it
    me, Mar 19, 2009
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