P0420 comes back after CAT and O2 sensors replaced

"Jeff Strickland" wrote in message

>
><snipped>

>>> I get HOW the sensor work, and what they are telling us. What I am
>>> saying is that the after CAT sensor is saying the CAT is not working
>>> right, but I maintain the CAT is fine and the after-CAT sensor itself is
>>> failing.
>>
>> The facts don't match what you maintain, therefore you ignore the facts?
>>
>>>
>>> The exhaust stream going into the CAT is okay, but the stream coming out
>>> is being reported as out of spec. If the stream is REPORTED as out of
>>> spec, then the problem can be eitherb the CAT itself, or the sensor
>>> making the report. The stream going into the CAT is within spec,
>>> therefore there is no point in checking system components that adjust
>>> the upstream exhaust contents.
>>>
>>
>> How do you KNOW that the exhaust going into the cat is within spec?
>>
>
> Because there is a sensor that says so. More accurately, there is a sensor
> that does not say the exhaust stream is out of spec.
>

Which sensor checks whether the exhaust stream going into the cat is in or
out of spec? What is it checking for?


>> You may think that there is no point in checking system components that
>> adjust the upstream exhaust components, however, the folks that designed
>> and built the system think differently and so list several components and
>> systems to check. I would trust their advice before I trusted your
>> advice or my advice. I am simply relaying what the factory says to
>> check.
>>
>
> I would trust them too, but in the face of not having all of the
> diagnostic tools at my disposal, I have to replace stuff at home using a
> Best Guess method. Since there are sensors before the CAT that are not
> giving an alarm, and there is a sensor after the CAT that is, AND I have
> faith that the CAT is a reliable component, I have to give my Best Guess
> on a failed sensor afgter the CAT. I would replace the sensor that is
> giving the alarm and hope the alarm goes off.
>

If one is going to attempt carpentry, one should have basic carpentry tools
on hand, including a tape measure to measure length. With a tape measure,
a carpenter can measure length instead of cutting boards until one fits. If
one is going to attempt automotive diagnostics, one should have basic
automotive tools on hand, including a volt/ohm meter to measure volts and
ohms. A perfectly servicable volt/ohm meter can be purchased at Radio Shack
for under $25. With a volt/ohm meter, one can measure voltage instead of
replacing parts until one works. That $25 investment in a volt/ohm meter
will save hundreds or even thousands of dollars by avoiding replacing good
parts. In my humble opinion, spending $25 on a meter that can be used for
checking just about every electrical component in a car is a reasonable
investment to help decide whether one should spend $50 on an aftermarket O2
sensor or even a $125 OEM sensor. Not only will you know whether or not the
sensor needs replacement, you will have a tool that will help check other
stuff in the future. I spent around $15 for my meter at Radio Shack almost
30 years ago, and as much as I would like a $400 Fluke meter, my cheapo
meter has never not worked for checking anything in a car.

Please think about and answer this a hypothetical question: You have 2
different O2 sensors, one in a blue car and one in a red car. The one in
the blue car is putting out a signal that meets factory specifications and
is rapidly cycling between 200 and 800 mV. The one in the red car is
putting out the same signal as the one in the blue car. Is the sensor in
the blue car good or bad?

To check your knowledge of how sensor #2 works, go to this link for on-line
test questions for the ASE test http://www.autoshop101.com/ and do questions
#7 and #17. The site lets you know whether your answers are correct or not.
In the scenario described in question #7, DTC P0420 would be set.

I don't expect everyone to know and understand the answer to question #7, so
in my desire to share my experience with and teach those who do not know, I
gave the CORRECT answer. My answer happens to agree with the National
Institute for Automotive Service Excellence, which is the body that tests
and certifies automotive technicians, with the factory serive manual, and I
suspect with Haynes, Chilton, and anyone else who actually understands how
DTC P0420 is set. Why do you insist in giving advice that is contrary to
the advice given by the factory and every legitimate source of automotive
information?
--

Ray O
(correct punctuation to reply)

wrote:

> It is raining today, on my way home, the check light turned on and I
> checked, it reads P0420. I just have CAT and 2 O2 sensors(front adn
> back) replaced about a month, how come?

Before replacing an O2 sensor, I would read their voltages, either
directly or through the car's computer. You'll need either an
oscilloscope (for electronics, not the kind for ignition systems) or a
digital voltage meter (don't use the analog type since it will load
down the sensor too much and slightly damage it). It's best if the
meter has a bar graph because that reacts much faster than the digits
display. The front O2 sensor should read about 1/2 volt AC on the
meter, or about 1 volt peak-peak on the oscilloscope. The rear sensor
should read the same if the catalytic converter doesn't work at all;
otherwise its voltage reading will be much lower, about 0.1V peak-peak.
You may be able to do a very rough check of the catalytic converter by
pointing an infrared thermometer at it. It should be hotter than the
exhaust pipe in front of it by at least tens of degrees.