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Car Forum / Nissan / Nissan Cars / August 2005

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obd2 codes - the acurracy thereof

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William H. Gatez - 03 Aug 2005 22:55 GMT
car is 2002 nissan sentra 5 speed with 65k miles.

check engine light was on for a while, so went to the local autozone and
they pulled a p0037, o2 sensor. they had none in stock, so went to another
autozone, had it checked again, and code was same, p0037.

looked in factory service manual, says code is for heated o2 sensor #2, bank
#1. at this point the though crossed my mind as to why the car needs 4 o2
sensors, but i digress.

ordered the correct sensor direct for bosch, the oem for this car ($100).
verified part numbers the same and installed. reset check engine light.

not too happy when it came on again after one trip. so either the
replacement o2 sensor is bad, or it wasn't the o2 sensor after all.

how accurate are these odbII codes supposed to be?
njmodi - 04 Aug 2005 02:53 GMT
The short answer (in my experience) is it depends.  For some sensors,
the ECU detects readings that are out of spec and pinpoints exact
sensors, e.g. the O2 sensor in your case.  There are several things
that can fail with an O2 sensor, the sensor itself could be giving bad
readings (out of range), the O2 sensor heating circuit can fail, etc.
The ECU is able to narrow down failures quite accurately in this case
and in the case of various other electronic sensors.

OTOH, for EGR systems for example, the ECU may report a malfunction
that represents a hi/low EGR flow.  The root cause of this code could
be one of several things, e.g. a bad EGR temp/flow sensor, a vacuum
leak to the EGR components, clogged EGR passges, a stuck  EGR valve,
etc.  So in this case further investigation is needed to accurately
determine the root cause.

To your specific case:

1. Are you sure you replaced the right sensor? (not undermining your
ability, but as you said, there are 4 sensors).
2. You could have perhaps contaminated the sensor with anti-seize.
3. Check the code - is it the same P0037 that has reposted.

Cheers,
Nirav
96 Max GLE, 120k
William H. Gatez - 04 Aug 2005 17:36 GMT
thanks for the reply. if there's one thing i'm sure of, i replaced the right
sensor. the book even has multiple pictures of the location (i gotta say
those factory manuals are pretty good). hell, i even went to sears and spent
$40 on a set of crowfoot wrenches so i could torque it properly. the sensor
came with anti-seize already on the threads, and a protective cap over the
threads/sensor part. i was careful not to smear it around.

i did not check to see if the code is the same - guess another trip to
autozone is in order.

btw, here's the text from the manual - you were right-on, sounds like the
heater itself failed?

The current amperage in the heated oxygen sensor 2
heater circuit is out of the normal range.
[An excessively high (P0038) or low (P0037) voltage signal
is sent to ECM through the heated oxygen sensor 2 heater.]

thanks,
jonathan

> The short answer (in my experience) is it depends.  For some sensors,
> the ECU detects readings that are out of spec and pinpoints exact
[quoted text clipped - 21 lines]
> Nirav
> 96 Max GLE, 120k
njmodi - 04 Aug 2005 19:37 GMT
Good to see you have the right book in front of you to guide you along.

Get the codes pulled again and post back with the results.

Not to ask the obvious - but you did plug in the new sensor's harness
right?

Nirav
larry - 04 Aug 2005 19:07 GMT
The codes are accurate. You could have a bad connector or bad wire going to
the sensors.

> car is 2002 nissan sentra 5 speed with 65k miles.
>
[quoted text clipped - 13 lines]
>
> how accurate are these odbII codes supposed to be?
William H. Gatez - 09 Aug 2005 19:05 GMT
had the code pulled again, same p0037.

checked continuity thru the sensor iaw the manual. it reads 2-4 ohms
where it should and open where it should. checked to see it was
getting 12v with ignition on - it is. checked to see it had a good
ground - it does. finally pulled the ecm and checked continuity from
the ecm cable connector thru the entire circuit path - good.

at this point i'm stumped. any other ideas appreciated.

thanks to all who replied.

>The codes are accurate. You could have a bad connector or bad wire going to
>the sensors.
[quoted text clipped - 17 lines]
>>
>> how accurate are these odbII codes supposed to be?
 
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