How to diagnose catalytic converter and O2 sensor?

My Honda Accord 98 (110,000mile) MIL lamp came on with code P0420
("Catalyst System Efficiency Below Threshold" ) occasionally about a
year ago. About 6 months ago, some wire around/in the Distributor
burned. My mechanic replaced the Distributor and Rotor, the MIL didn't
came back for 2~3 months. After that, the MIL started coming on and
staying on. I cleared the code for several times and it always came on
back after about 40 miles of drive. My car's mileage didn't change
which is about 30m/g.

Last week my car just failed state emssion test due to this MIL lamp. I
have a PC-based OBD-II scan tool but never used it before. Can somebody
tell me how can I use it to diagnose the problem? Follow are some
newbie questions:

1. How to measure O2 sensor's voltage? Can I trust the data polled from
ECU, or should physically measure it under the car? For those guys who
don't have scan tool, how did they measure it?

2. How do I tell it's secondary o2 sensor's problem or the convertor's?


3. Honda Service manual's instruction for P0420 is very simple: Do a
P0420 test first. If it failed, replace convertor. My local dealer
gonna charge me $1200 for this work. However, many on-line articles say
the culprit can be something else. I wonder if somebody can kindly tell
me the steps I can follow to pin point the issue?
4. Will replacment of the Rotor help?

Thanks!

-D

Dick wrote:
> My Honda Accord 98 (110,000mile) MIL lamp came on with code P0420
> ("Catalyst System Efficiency Below Threshold" ) occasionally about a
> year ago. About 6 months ago, some wire around/in the Distributor
> burned. My mechanic replaced the Distributor and Rotor, the MIL didn't
> came back for 2~3 months. After that, the MIL started coming on and
> staying on. I cleared the code for several times and it always came on
> back after about 40 miles of drive. My car's mileage didn't change
> which is about 30m/g.
>
> Last week my car just failed state emssion test due to this MIL lamp. I
> have a PC-based OBD-II scan tool but never used it before. Can somebody
> tell me how can I use it to diagnose the problem? Follow are some
> newbie questions:
>
> 1. How to measure O2 sensor's voltage? Can I trust the data polled from
> ECU, or should physically measure it under the car? For those guys who
> don't have scan tool, how did they measure it?

ohmmeter. i dont know the values offhand, but in my case (98 civic cx)
it told me to check the bottom 2 heater wires on the top O2 sensor. the
code wasnt a P0420, it was something like P0135, primary O2 sensor
heater malfunction, IIRC

but yeah- the helm manual (which is wayyyy less than $1200) will have
the values and procedures. all you need is that and an ohmmeter. mines a
$3 one from harbor freight.
>
> 2. How do I tell it's secondary o2 sensor's problem or the convertor's?

if youve been using "top tier" (chevron, union76, other name brand) from
a reputable station and have done all the required maintenance like
spark plug changes, etc... its almost definately the O2 sensor.
>
>
> 3. Honda Service manual's instruction for P0420 is very simple: Do a
> P0420 test first. If it failed, replace convertor. My local dealer
> gonna charge me $1200 for this work. However, many on-line articles say
> the culprit can be something else. I wonder if somebody can kindly tell
> me the steps I can follow to pin point the issue?

the service manual should tell exactly how to test it. i dont see how it
could cost $1200, unless theyre replacing the cat and sensor.

> 4. Will replacment of the Rotor help?

probably not, but wouldnt hurt if its never been done.
>
> Thanks!
>
> -D
>

SoCalMike wrote:
> Dick wrote:
> > My Honda Accord 98 (110,000mile) MIL lamp came on with code P0420
> > ("Catalyst System Efficiency Below Threshold" ) occasionally about
a
> > year ago. About 6 months ago, some wire around/in the Distributor
> > burned. My mechanic replaced the Distributor and Rotor, the MIL
didn't
> > came back for 2~3 months. After that, the MIL started coming on and
> > staying on. I cleared the code for several times and it always came
on
> > back after about 40 miles of drive. My car's mileage didn't change
> > which is about 30m/g.
> >
> > Last week my car just failed state emssion test due to this MIL
lamp. I
> > have a PC-based OBD-II scan tool but never used it before. Can
somebody
> > tell me how can I use it to diagnose the problem? Follow are some
> > newbie questions:
> >
> > 1. How to measure O2 sensor's voltage? Can I trust the data polled
from
> > ECU, or should physically measure it under the car? For those guys
who
> > don't have scan tool, how did they measure it?
>
> ohmmeter. i dont know the values offhand, but in my case (98 civic
cx)
> it told me to check the bottom 2 heater wires on the top O2 sensor.
the
> code wasnt a P0420, it was something like P0135, primary O2 sensor
> heater malfunction, IIRC
>
> but yeah- the helm manual (which is wayyyy less than $1200) will have

> the values and procedures. all you need is that and an ohmmeter.
mines a
> $3 one from harbor freight.
> >

Thanks! Sometimes it's needed to verify the o2 sensor behaves correctly
in certain driving patterns, which means the voltage has to measured
while the car is running. How do you measure it in this scenario?

> > 2. How do I tell it's secondary o2 sensor's problem or the
convertor's?
>
> if youve been using "top tier" (chevron, union76, other name brand)
from
> a reputable station and have done all the required maintenance like
> spark plug changes, etc... its almost definately the O2 sensor.

It might be. But, since I have an OBD-II scan tool handy, how can I get
a more convincing answer using the tool?

> >
> >
> > 3. Honda Service manual's instruction for P0420 is very simple: Do
a
> > P0420 test first. If it failed, replace convertor. My local dealer
> > gonna charge me $1200 for this work. However, many on-line articles
say
> > the culprit can be something else. I wonder if somebody can kindly
tell
> > me the steps I can follow to pin point the issue?
>
> the service manual should tell exactly how to test it. i dont see how
it
> could cost $1200, unless theyre replacing the cat and sensor.
>

Unfortunately, the service manual's diagnosis procedure is really that
simple as I said.

> > 4. Will replacment of the Rotor help?
>
> probably not, but wouldnt hurt if its never been done.
I just replaced it 6 months ago.

> >
> > Thanks!
> >
> > -D
> >

Dick wrote:
> It might be. But, since I have an OBD-II scan tool handy, how can I get
> a more convincing answer using the tool?

no idea. i plugged mine in, and it said something like

P0135- heated primary oxygen sensor heater circuit malfunction

(or something like that)

does the manual tell how to do a P0420 test?

On 26 Jan 2005 10:18:01 -0800, "Dick" > wrote:

>SoCalMike wrote:
>> Dick wrote:
>> > My Honda Accord 98 (110,000mile) MIL lamp came on with code P0420
>> > ("Catalyst System Efficiency Below Threshold" ) occasionally about
>a
>> > year ago. About 6 months ago, some wire around/in the Distributor
>> > burned. My mechanic replaced the Distributor and Rotor, the MIL
>didn't
>> > came back for 2~3 months. After that, the MIL started coming on and
>> > staying on. I cleared the code for several times and it always came
>on
>> > back after about 40 miles of drive. My car's mileage didn't change
>> > which is about 30m/g.
>> >
>> > Last week my car just failed state emssion test due to this MIL
>lamp. I
>> > have a PC-based OBD-II scan tool but never used it before. Can
>somebody
>> > tell me how can I use it to diagnose the problem? Follow are some
>> > newbie questions:
>> >
>> > 1. How to measure O2 sensor's voltage? Can I trust the data polled
>from
>> > ECU, or should physically measure it under the car? For those guys
>who
>> > don't have scan tool, how did they measure it?
>>
>> ohmmeter. i dont know the values offhand, but in my case (98 civic
>cx)
>> it told me to check the bottom 2 heater wires on the top O2 sensor.
>the
>> code wasnt a P0420, it was something like P0135, primary O2 sensor
>> heater malfunction, IIRC
>>
>> but yeah- the helm manual (which is wayyyy less than $1200) will have
>
>> the values and procedures. all you need is that and an ohmmeter.
>mines a
>> $3 one from harbor freight.
>> >
>
>Thanks! Sometimes it's needed to verify the o2 sensor behaves correctly
>in certain driving patterns, which means the voltage has to measured
>while the car is running. How do you measure it in this scenario?
>
>> > 2. How do I tell it's secondary o2 sensor's problem or the
>convertor's?
>>
>> if youve been using "top tier" (chevron, union76, other name brand)
>from
>> a reputable station and have done all the required maintenance like
>> spark plug changes, etc... its almost definately the O2 sensor.
>
>It might be. But, since I have an OBD-II scan tool handy, how can I get
>a more convincing answer using the tool?
>
>> >
>> >
>> > 3. Honda Service manual's instruction for P0420 is very simple: Do
>a
>> > P0420 test first. If it failed, replace convertor. My local dealer
>> > gonna charge me $1200 for this work. However, many on-line articles
>say
>> > the culprit can be something else. I wonder if somebody can kindly
>tell
>> > me the steps I can follow to pin point the issue?
>>
>> the service manual should tell exactly how to test it. i dont see how
>it
>> could cost $1200, unless theyre replacing the cat and sensor.
>>
>
>Unfortunately, the service manual's diagnosis procedure is really that
>simple as I said.
>
>> > 4. Will replacment of the Rotor help?
>>
>> probably not, but wouldnt hurt if its never been done.
>I just replaced it 6 months ago.
>
>> >
>> > Thanks!
>> >
>> > -D
>> >


it's a bad cat. common at this mileage. just replace it. i see them
all the time on this model.
Chip

OK, Here it goes 02 class 101 (and catalyst efficiency)


The O2 sensor generates its own voltage, and without my meter I couldn't
tell you, BUT trust your scanner if it gives you a voltage reading. The
first o2 sensor switches when the engines running, you watch it go low to
high (sometimes .020 volts to a high of .900 volts (Milivolts??)
It should click along at a steady pace, picking random numbers into the
700-900 range then low again in the 100's
It is looking for Oxygen. This sensors job is to tune the car if it starts
to run to rich; i.e., if too much fuel starts feeding into the cylinders,
for whatever reason, the o2 sensor will start showing a higher range of
numbers, and the computer will start decreasing the injector pulse to
decrease the amount of fuel.
Anyway, a 02 sensor can get sluggish or it can freeze at a random number
then it lies to the computer that will still try to correct it. but usually
this sets a 02 code...

Now that basically describes the 02 sensors operation. The 2nd o2 sensor's
job is to check to see if the cat converter is doing it's job (Generally,
most 02 sensors are the exact same part excepting for the plug and wire
length). It also switches the same as the first one but at a slower or
reduced rate. If your scan tool shows the aft 02 sensor stuck, I'd try it
first.
If your second (aft) 02 sensor is switching at the same pace as the first o2
sensor (the numbers won't match) then chances are the cat is bad.




--
Stephen W. Hansen
ASE Certified Master Automobile Technician
ASE Undercar Specialist













"Dick" > wrote in message
oups.com...
> SoCalMike wrote:
>> Dick wrote:
>> > My Honda Accord 98 (110,000mile) MIL lamp came on with code P0420
>> > ("Catalyst System Efficiency Below Threshold" ) occasionally about
> a
>> > year ago. About 6 months ago, some wire around/in the Distributor
>> > burned. My mechanic replaced the Distributor and Rotor, the MIL
> didn't
>> > came back for 2~3 months. After that, the MIL started coming on and
>> > staying on. I cleared the code for several times and it always came
> on
>> > back after about 40 miles of drive. My car's mileage didn't change
>> > which is about 30m/g.
>> >
>> > Last week my car just failed state emssion test due to this MIL
> lamp. I
>> > have a PC-based OBD-II scan tool but never used it before. Can
> somebody
>> > tell me how can I use it to diagnose the problem? Follow are some
>> > newbie questions:
>> >
>> > 1. How to measure O2 sensor's voltage? Can I trust the data polled
> from
>> > ECU, or should physically measure it under the car? For those guys
> who
>> > don't have scan tool, how did they measure it?
>>
>> ohmmeter. i dont know the values offhand, but in my case (98 civic
> cx)
>> it told me to check the bottom 2 heater wires on the top O2 sensor.
> the
>> code wasnt a P0420, it was something like P0135, primary O2 sensor
>> heater malfunction, IIRC
>>
>> but yeah- the helm manual (which is wayyyy less than $1200) will have
>
>> the values and procedures. all you need is that and an ohmmeter.
> mines a
>> $3 one from harbor freight.
>> >
>
> Thanks! Sometimes it's needed to verify the o2 sensor behaves correctly
> in certain driving patterns, which means the voltage has to measured
> while the car is running. How do you measure it in this scenario?
>
>> > 2. How do I tell it's secondary o2 sensor's problem or the
> convertor's?
>>
>> if youve been using "top tier" (chevron, union76, other name brand)
> from
>> a reputable station and have done all the required maintenance like
>> spark plug changes, etc... its almost definately the O2 sensor.
>
> It might be. But, since I have an OBD-II scan tool handy, how can I get
> a more convincing answer using the tool?
>
>> >
>> >
>> > 3. Honda Service manual's instruction for P0420 is very simple: Do
> a
>> > P0420 test first. If it failed, replace convertor. My local dealer
>> > gonna charge me $1200 for this work. However, many on-line articles
> say
>> > the culprit can be something else. I wonder if somebody can kindly
> tell
>> > me the steps I can follow to pin point the issue?
>>
>> the service manual should tell exactly how to test it. i dont see how
> it
>> could cost $1200, unless theyre replacing the cat and sensor.
>>
>
> Unfortunately, the service manual's diagnosis procedure is really that
> simple as I said.
>
>> > 4. Will replacment of the Rotor help?
>>
>> probably not, but wouldnt hurt if its never been done.
> I just replaced it 6 months ago.
>
>> >
>> > Thanks!
>> >
>> > -D
>> >
>

If the O2 sensor has been bad for six years your Cat is toast. To learn how
to use the OBD2 tester read the manual and use it. There is no first grade
instruction steps to using it. You have to use your grey matter. For
information on diagnosing problems read here
http://www.autotap.com/obdii_library.asp
If you can't read try your local elementary school...


"Steve" > wrote in message
...
>
> OK, Here it goes 02 class 101 (and catalyst efficiency)
>
>
> The O2 sensor generates its own voltage, and without my meter I couldn't
> tell you, BUT trust your scanner if it gives you a voltage reading. The
> first o2 sensor switches when the engines running, you watch it go low to
> high (sometimes .020 volts to a high of .900 volts (Milivolts??)
> It should click along at a steady pace, picking random numbers into the
> 700-900 range then low again in the 100's
> It is looking for Oxygen. This sensors job is to tune the car if it starts
> to run to rich; i.e., if too much fuel starts feeding into the cylinders,
> for whatever reason, the o2 sensor will start showing a higher range of
> numbers, and the computer will start decreasing the injector pulse to
> decrease the amount of fuel.
> Anyway, a 02 sensor can get sluggish or it can freeze at a random number
> then it lies to the computer that will still try to correct it. but
> usually this sets a 02 code...
>
> Now that basically describes the 02 sensors operation. The 2nd o2 sensor's
> job is to check to see if the cat converter is doing it's job (Generally,
> most 02 sensors are the exact same part excepting for the plug and wire
> length). It also switches the same as the first one but at a slower or
> reduced rate. If your scan tool shows the aft 02 sensor stuck, I'd try it
> first.
> If your second (aft) 02 sensor is switching at the same pace as the first
> o2 sensor (the numbers won't match) then chances are the cat is bad.
>
>
>
>
> --
> Stephen W. Hansen
> ASE Certified Master Automobile Technician
> ASE Undercar Specialist
>
>
>
>
>
>
>
>
>
>
>
>
>
> "Dick" > wrote in message
> oups.com...
>> SoCalMike wrote:
>>> Dick wrote:
>>> > My Honda Accord 98 (110,000mile) MIL lamp came on with code P0420
>>> > ("Catalyst System Efficiency Below Threshold" ) occasionally about
>> a
>>> > year ago. About 6 months ago, some wire around/in the Distributor
>>> > burned. My mechanic replaced the Distributor and Rotor, the MIL
>> didn't
>>> > came back for 2~3 months. After that, the MIL started coming on and
>>> > staying on. I cleared the code for several times and it always came
>> on
>>> > back after about 40 miles of drive. My car's mileage didn't change
>>> > which is about 30m/g.
>>> >
>>> > Last week my car just failed state emssion test due to this MIL
>> lamp. I
>>> > have a PC-based OBD-II scan tool but never used it before. Can
>> somebody
>>> > tell me how can I use it to diagnose the problem? Follow are some
>>> > newbie questions:
>>> >
>>> > 1. How to measure O2 sensor's voltage? Can I trust the data polled
>> from
>>> > ECU, or should physically measure it under the car? For those guys
>> who
>>> > don't have scan tool, how did they measure it?
>>>
>>> ohmmeter. i dont know the values offhand, but in my case (98 civic
>> cx)
>>> it told me to check the bottom 2 heater wires on the top O2 sensor.
>> the
>>> code wasnt a P0420, it was something like P0135, primary O2 sensor
>>> heater malfunction, IIRC
>>>
>>> but yeah- the helm manual (which is wayyyy less than $1200) will have
>>
>>> the values and procedures. all you need is that and an ohmmeter.
>> mines a
>>> $3 one from harbor freight.
>>> >
>>
>> Thanks! Sometimes it's needed to verify the o2 sensor behaves correctly
>> in certain driving patterns, which means the voltage has to measured
>> while the car is running. How do you measure it in this scenario?
>>
>>> > 2. How do I tell it's secondary o2 sensor's problem or the
>> convertor's?
>>>
>>> if youve been using "top tier" (chevron, union76, other name brand)
>> from
>>> a reputable station and have done all the required maintenance like
>>> spark plug changes, etc... its almost definately the O2 sensor.
>>
>> It might be. But, since I have an OBD-II scan tool handy, how can I get
>> a more convincing answer using the tool?
>>
>>> >
>>> >
>>> > 3. Honda Service manual's instruction for P0420 is very simple: Do
>> a
>>> > P0420 test first. If it failed, replace convertor. My local dealer
>>> > gonna charge me $1200 for this work. However, many on-line articles
>> say
>>> > the culprit can be something else. I wonder if somebody can kindly
>> tell
>>> > me the steps I can follow to pin point the issue?
>>>
>>> the service manual should tell exactly how to test it. i dont see how
>> it
>>> could cost $1200, unless theyre replacing the cat and sensor.
>>>
>>
>> Unfortunately, the service manual's diagnosis procedure is really that
>> simple as I said.
>>
>>> > 4. Will replacment of the Rotor help?
>>>
>>> probably not, but wouldnt hurt if its never been done.
>> I just replaced it 6 months ago.
>>
>>> >
>>> > Thanks!
>>> >
>>> > -D
>>> >
>>
>
>

Thanks, Steve! Your class really helps!

Woody, thanks for the autotap link! I will check that out.

By the way, why you think my O2 sensor has been bad for 6 years? And
where did you get the impression that I need help with my scan tool? If
I need go to an elementary school I will grab you and go there
together.