• PO420 Catalyst System Efficiency Below Threshold
  • PO420 Catalyst System Efficiency Below Threshold
  • PO420 Catalyst System Efficiency Below Threshold
  • PO420 Catalyst System Efficiency Below Threshold

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  1. #1
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    Default PO420 Catalyst System Efficiency Below Threshold

    I am getting a PO420 Catalyst System Efficiency Below Threshold trouble code and have a few questions. I got the code last week and used my scanner to test the 02 sensors and the diagnostics of the car itself. I then deleted the code and a week later the same code came up again.

    Here are the numbers I got from my scanner:


    Here are some other numbers that I have... One with the engine at idle, and then 47% load.

    http://memimage.cardomain.com/ride_i...0251_large.jpg (Idle)

    http://memimage.cardomain.com/ride_i...0250_large.jpg (47% load)



    What does everyone think? Bad O2 sensor in bank one, or bad Cat, or something else?

    The service manual says to change the O2 sensor you need to drop the exhaust? Does anyone have any experience?

    Also, bank 1 is the drivers side or passenger side? how do I know if the pre cat sensor is bad or the sensor after the cat?

    Thanks!

  2. #2
    MBDiagMan
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    This is typically telling you that you have a bad cat. The system looks for a 100 degree difference between input and output cat temperature. When it is less than that, something is up with the cat.

  3. #3
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    Quote Originally Posted by MBDiagMan View Post
    This is typically telling you that you have a bad cat. The system looks for a 100 degree difference between input and output cat temperature. When it is less than that, something is up with the cat.
    I'm sorry but the above is not correct.

    The engine controls cannot measure exhaust temperature.

    What the engine controls look at is the signals from the front and rear oxygen sensors (O2S).

    When the rear sensor signal becomes similar to or begins to mimic the front sensor, the ECM turns on the MIL. The "assumption" is that when the rear signal mimics the front signal, the cat is either not there, has failed or is near failure.

    If you are getting repeated codes for low cat efficiency, it's a very good bet that the cat related to that code has gone bad.

    It is true that measuring the temperature of the pipe going into the cat and going out of the cat sometimes can confirm that but in/out pipe temps are not always an accurate method of assessing cat condition.

    The only accurate methods are comparing the O2S signals or, even better, drilling the exhaust ahead of the cat then comparing the results of a test with a five-gas analyzer.

  4. #4
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    Do the readings from the O2 sensors look right? Im new to this scanner thing and I am not completely positive what I am looking for... I am hoping it is just an O2 sensor, but how can I tell if it is that or the cat?

    Thanks for the replies!

  5. #5
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    If the code that is set is, in fact, P0240, it's possible but unlikely that the problem is an O2S.

    What I'd do is get out your Factory Service Manual, and follow the diagnostic information for that code.

    My guess is that it's going to lead you to a bad cat.

    How many miles on this engine?
    Has it ever been run on leaded gas?
    Has it ever been run on gasoline mixed with an octane booster containing MMT?

  6. #6
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    All 4 O2 sensors on your 96 are the same, instead of buying one remove the suspect sensor and swap with a different one, clear the code and see if it returns or moves to the new location. If you get the same code you likely have a bad cat, if it moves replace the sensor.

  7. #7
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    I got the Vette this December with 67K miles on it and now it has 70K... It was a collectors car, and cared for very very well! I dont think there were ever any additives added to the gas, or that it was ever run on leaded fuel. It was in NJ and MD most of its life, so...

    I will read the service manuals and follow the directions, but I am hoping it is not a bad Cat! They are pretty expensive.
    If it is bank one then it would be the drivers side Cat right?

    Thanks for the advice everyone!

  8. #8
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    You're correct.. bank 1 is drivers side....a far as the P0420--since you have no other codes such as a problem dumping so much fuel the converter couldn't do its job which would also set a rich/lean code at the very least.

    I've always thought of the sensor behind the converter as a sort of "tattle tale" on whether or not the converter is doing it's job. The ecm makes fuel trim adjusments according to what sensor 1 (in front of the cat "sees")---if the cat is working correctly sensor 2 really doesn't do much/the ecm sees very little activity from sensor 2---if the ecm sees a lot of activity from sensor 2----then the converter is NOT doing it's job and P0420 will be set. I feel you'll be replacing a cat.

  9. #9
    MBDiagMan
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    Quote Originally Posted by Hib Halverson View Post
    I'm sorry but the above is not correct.

    The engine controls cannot measure exhaust temperature.

    What the engine controls look at is the signals from the front and rear oxygen sensors (O2S).

    When the rear sensor signal becomes similar to or begins to mimic the front sensor, the ECM turns on the MIL. The "assumption" is that when the rear signal mimics the front signal, the cat is either not there, has failed or is near failure.

    If you are getting repeated codes for low cat efficiency, it's a very good bet that the cat related to that code has gone bad.

    It is true that measuring the temperature of the pipe going into the cat and going out of the cat sometimes can confirm that but in/out pipe temps are not always an accurate method of assessing cat condition.

    The only accurate methods are comparing the O2S signals or, even better, drilling the exhaust ahead of the cat then comparing the results of a test with a five-gas analyzer.
    So just what in the H$++ do you think that Lambda sensors measure, vehicle speed?

    The sensors are putting out a voltage from which temperature can be derived. Yes, the code is tripped by the fact that in and out are nearly the same, but the idea is to measure the in and out and compare.

    BTW, if I'm wrong, I think that you need to contact ASE and tell them that their test question is incorrect.

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    Quote Originally Posted by MBDiagMan View Post
    So just what in the H$++ do you think that Lambda sensors measure, vehicle speed?

    The sensors are putting out a voltage from which temperature can be derived. Yes, the code is tripped by the fact that in and out are nearly the same, but the idea is to measure the in and out and compare.

    BTW, if I'm wrong, I think that you need to contact ASE and tell them that their test question is incorrect.
    You may want to review O2 operation before it is time to recertify, they measure concentration of oxygen, not temperature.

    How Does the Oxygen Sensor Work

  11. #11
    MBDiagMan
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    Yes they are measuring Oxygen presence, but it is BASED ON TEMPERATURE. It's sort of like MEASURING horsepower. Horsepower is not measured, it is calculated based on torque and RPM to derive horsepower.

    The Oxygen level is derived from the value from the Lambda Sensor which in automotive use is called an Oxygen sensor. It generates a Voltage that is determined by the temperature it senses. That is then used to DERIVE oxygen content.

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    Quote Originally Posted by MBDiagMan View Post
    Yes they are measuring Oxygen presence, but it is BASED ON TEMPERATURE. It's sort of like MEASURING horsepower. Horsepower is not measured, it is calculated based on torque and RPM to derive horsepower.

    The Oxygen level is derived from the value from the Lambda Sensor which in automotive use is called an Oxygen sensor. It generates a Voltage that is determined by the temperature it senses. That is then used to DERIVE oxygen content.
    First time I've heard that theory, any evidence to support it? The sirconia style O2 sensor used in nearly all American cars functions by electrochemical process similar to a battery.

    Oxygen sensor - Wikipedia, the free encyclopedia

  13. #13
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    Quote Originally Posted by toptechx6 View Post
    First time I've heard that theory, any evidence to support it? The sirconia style O2 sensor used in nearly all American cars functions by electrochemical process similar to a battery.

    Oxygen sensor - Wikipedia, the free encyclopedia
    No he doesn't have any evidence--that's his own personal-one of a kind-very unique definition of lambda/O2 sensor----What he needs to do is go to the library or online and find the definition of lambda sensor. The only thing a o2 sensor needs as far as temperature goes IS it needs to be HOT in order to function correctly. As "Top" said--- if I was you I'd brush up on design and function of the o2 sensor before recertification rolls around.

  14. #14
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    Motor Age A8 Engine Performance, page 54: "With the engine at normal operating temperature, check the temperature of the exhaust inlet and outlet surface before and after the converter using a temperature probe and a DMM or an asxhuast pyrometer. The exhaust surface temperature shoud be at least 100 degrees F hotter than the intake surface temperature. If not the converter is probably not operating at peak efficiency."

    Same publication, page 62: "..... Some vehicles use more than one sensor. OBDII systems use a second sensor downstream of the converter to check its efficiency."


    So guys, you can belittle my knowledge all you like regarding the exact way that a Lambda sensor produces a voltage, but they are still used to check converter efficiency.

    Also, I appreciate very much your concern for my ability to pass my recertifications, but I can assure you that it will be no problem whatsoever.

    Now if you insist on continuing to use the ROTFLMAO moniker to belittle me, help yourself if it makes you feel better. Weak people very commonly build themselves up by putting others down.

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    Quote Originally Posted by MBDiagMan View Post
    Motor Age A8 Engine Performance, page 54: "With the engine at normal operating temperature, check the temperature of the exhaust inlet and outlet surface before and after the converter using a temperature probe and a DMM or an asxhuast pyrometer. The exhaust surface temperature shoud be at least 100 degrees F hotter than the intake surface temperature. If not the converter is probably not operating at peak efficiency."

    Same publication, page 62: "..... Some vehicles use more than one sensor. OBDII systems use a second sensor downstream of the converter to check its efficiency."


    So guys, you can belittle my knowledge all you like regarding the exact way that a Lambda sensor produces a voltage, but they are still used to check converter efficiency.

    Also, I appreciate very much your concern for my ability to pass my recertifications, but I can assure you that it will be no problem whatsoever.

    Now if you insist on continuing to use the ROTFLMAO moniker to belittle me, help yourself if it makes you feel better. Weak people very commonly build themselves up by putting others down.
    Yes sir-- now that's very informative and it's got a lot of info on how a O2 sensor works---that info was great back in 1973 for checking a converter-- before the use of O2 sensors which were placed before and after the cat--it serves no purpose today. For anyone who reads this thread completely and actually KNOWS how an O2 sensor works----no one here has to"belittle your knowledge"---you've done that YOURSELF simply by posting

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