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

 

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.

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.

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.

probably not, but wouldnt hurt if its never been done.

 

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?

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

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

I just replaced it 6 months ago.

 

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?

 

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

 

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...

 

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.