Jeff Strickland

"DougW" <post.replies@invalid.address> wrote in message
news:d8drh.41182$oA1.30083@newsfe19.lga...
> Jeff Strickland wrote:
>> "bllsht" wrote ...
>>> "DougW" wrote:
>>>
>>>> bspear78 wrote:
>>>>> I have a check engine light on, and the code is reading a 12 and a
>>>>> 72. I know what the 12 means, but could not find any explanation
>>>>> for a reading of 72. Anyone know what this means?
>>>>
>>>> 72 Catalyst efficiency below required level. (Same as code 64)
>>>>
>>>> This means one of two things.
>>>> 1) your catalytic converter is plugging/cracking/failing
>>>> 2) the O2 sensor behind the cat is failing
>>>
>>> If the downstream O2 was failing you'd get a downstream O2 fault, not
>>> a cat efficiency fault.
>>>
>>
>> I'm not sure that is true in an OBD I car. The CAT is not a likely
>> source of problems, and my money is on the after-CAT sensor going bad.
>>
>> I am pretty sure that the downstream sensor can give an error that
>> says the CAT is failing. This can happen because the system doesn't
>> know that the sensor is bad, it only knows that the input from the
>> sensor is not correct. The sensor can fail in a mode that makes the
>> system say that the CAT is not working when the reality is that the
>> sensor itself has taken a bye.
>> Having said all of that, the OP says he has a 97 YJ. In the 97, it was
>> called a TJ, and in 97, the codes are OBD II.
>
> I've seen O2 sensors go bad, but not bad enough to set a fault. Usually
> it's the forward one and the only thing it does is give you bad milage
> and black exhaust. Figured the rearward one could go equally bad and
> throw a code even if the cat was good.
>


I agree, usually it's the front sensor. But, the CAT's condition is not
checked at the front, it's checked at the back.

The "front" (upstream) sensor will adjust the fuel mix as much as it can,
and eventually throw a code that results from detecting an out of range
condition (rich or lean mixture) that the computer is not able to adjust
fuel delivery to correct for. The out of range can be true or false. If
true, the cause can be related to stuff associated with fuel delivery --
faulty injectors, that sort of thing -- but if false, then the trouble is a
faulty sensor.

The "back" (downstream) sensor is looking at what comes out of the CAT. It
can also give a true or false report. If the report is true, the cause is a
faulty CAT, but if false, the cause is a faulty sensor. Given the
environment (hot exhaust stream), I tend to look at faulty sensors,
especially when the CAT is reported as the trouble. The CAT itself is very
robust, and is capable of lasting nearly as long as the vehicle it is
mounted to. The sensor is not so robust.

If the code says that the CAT is below efficiency, I'd be thinking the
sensor that reports the CAT is what is really below efficiency. If the
problem was happening at the upstream sensor, the report would not be that
the CAT is bad, because such data would be collected before the CAT comes
into play. A report from the upstream sensor would include information that
the fuel mixture was 1.) too rich, 2.) too lean, or 3.) out of range,
without an indication as to the content of the mixture. The downstream
sensor does not know about to rich or too lean, it only knows that what
comes out of the CAT has not been cleaned up, relative to what went in. The
upstream sensor knows what goes into the CAT, the downstream sensor knows
what comes out.

Jeff Strickland

"DougW" <post.replies@invalid.address> wrote in message
news:d8drh.41182$oA1.30083@newsfe19.lga...
> Jeff Strickland wrote:
>> "bllsht" wrote ...
>>> "DougW" wrote:
>>>
>>>> bspear78 wrote:
>>>>> I have a check engine light on, and the code is reading a 12 and a
>>>>> 72. I know what the 12 means, but could not find any explanation
>>>>> for a reading of 72. Anyone know what this means?
>>>>
>>>> 72 Catalyst efficiency below required level. (Same as code 64)
>>>>
>>>> This means one of two things.
>>>> 1) your catalytic converter is plugging/cracking/failing
>>>> 2) the O2 sensor behind the cat is failing
>>>
>>> If the downstream O2 was failing you'd get a downstream O2 fault, not
>>> a cat efficiency fault.
>>>
>>
>> I'm not sure that is true in an OBD I car. The CAT is not a likely
>> source of problems, and my money is on the after-CAT sensor going bad.
>>
>> I am pretty sure that the downstream sensor can give an error that
>> says the CAT is failing. This can happen because the system doesn't
>> know that the sensor is bad, it only knows that the input from the
>> sensor is not correct. The sensor can fail in a mode that makes the
>> system say that the CAT is not working when the reality is that the
>> sensor itself has taken a bye.
>> Having said all of that, the OP says he has a 97 YJ. In the 97, it was
>> called a TJ, and in 97, the codes are OBD II.
>
> I've seen O2 sensors go bad, but not bad enough to set a fault. Usually
> it's the forward one and the only thing it does is give you bad milage
> and black exhaust. Figured the rearward one could go equally bad and
> throw a code even if the cat was good.
>


I agree, usually it's the front sensor. But, the CAT's condition is not
checked at the front, it's checked at the back.

The "front" (upstream) sensor will adjust the fuel mix as much as it can,
and eventually throw a code that results from detecting an out of range
condition (rich or lean mixture) that the computer is not able to adjust
fuel delivery to correct for. The out of range can be true or false. If
true, the cause can be related to stuff associated with fuel delivery --
faulty injectors, that sort of thing -- but if false, then the trouble is a
faulty sensor.

The "back" (downstream) sensor is looking at what comes out of the CAT. It
can also give a true or false report. If the report is true, the cause is a
faulty CAT, but if false, the cause is a faulty sensor. Given the
environment (hot exhaust stream), I tend to look at faulty sensors,
especially when the CAT is reported as the trouble. The CAT itself is very
robust, and is capable of lasting nearly as long as the vehicle it is
mounted to. The sensor is not so robust.

If the code says that the CAT is below efficiency, I'd be thinking the
sensor that reports the CAT is what is really below efficiency. If the
problem was happening at the upstream sensor, the report would not be that
the CAT is bad, because such data would be collected before the CAT comes
into play. A report from the upstream sensor would include information that
the fuel mixture was 1.) too rich, 2.) too lean, or 3.) out of range,
without an indication as to the content of the mixture. The downstream
sensor does not know about to rich or too lean, it only knows that what
comes out of the CAT has not been cleaned up, relative to what went in. The
upstream sensor knows what goes into the CAT, the downstream sensor knows
what comes out.

Jeff Strickland

"DougW" <post.replies@invalid.address> wrote in message
news:d8drh.41182$oA1.30083@newsfe19.lga...
> Jeff Strickland wrote:
>> "bllsht" wrote ...
>>> "DougW" wrote:
>>>
>>>> bspear78 wrote:
>>>>> I have a check engine light on, and the code is reading a 12 and a
>>>>> 72. I know what the 12 means, but could not find any explanation
>>>>> for a reading of 72. Anyone know what this means?
>>>>
>>>> 72 Catalyst efficiency below required level. (Same as code 64)
>>>>
>>>> This means one of two things.
>>>> 1) your catalytic converter is plugging/cracking/failing
>>>> 2) the O2 sensor behind the cat is failing
>>>
>>> If the downstream O2 was failing you'd get a downstream O2 fault, not
>>> a cat efficiency fault.
>>>
>>
>> I'm not sure that is true in an OBD I car. The CAT is not a likely
>> source of problems, and my money is on the after-CAT sensor going bad.
>>
>> I am pretty sure that the downstream sensor can give an error that
>> says the CAT is failing. This can happen because the system doesn't
>> know that the sensor is bad, it only knows that the input from the
>> sensor is not correct. The sensor can fail in a mode that makes the
>> system say that the CAT is not working when the reality is that the
>> sensor itself has taken a bye.
>> Having said all of that, the OP says he has a 97 YJ. In the 97, it was
>> called a TJ, and in 97, the codes are OBD II.
>
> I've seen O2 sensors go bad, but not bad enough to set a fault. Usually
> it's the forward one and the only thing it does is give you bad milage
> and black exhaust. Figured the rearward one could go equally bad and
> throw a code even if the cat was good.
>


I agree, usually it's the front sensor. But, the CAT's condition is not
checked at the front, it's checked at the back.

The "front" (upstream) sensor will adjust the fuel mix as much as it can,
and eventually throw a code that results from detecting an out of range
condition (rich or lean mixture) that the computer is not able to adjust
fuel delivery to correct for. The out of range can be true or false. If
true, the cause can be related to stuff associated with fuel delivery --
faulty injectors, that sort of thing -- but if false, then the trouble is a
faulty sensor.

The "back" (downstream) sensor is looking at what comes out of the CAT. It
can also give a true or false report. If the report is true, the cause is a
faulty CAT, but if false, the cause is a faulty sensor. Given the
environment (hot exhaust stream), I tend to look at faulty sensors,
especially when the CAT is reported as the trouble. The CAT itself is very
robust, and is capable of lasting nearly as long as the vehicle it is
mounted to. The sensor is not so robust.

If the code says that the CAT is below efficiency, I'd be thinking the
sensor that reports the CAT is what is really below efficiency. If the
problem was happening at the upstream sensor, the report would not be that
the CAT is bad, because such data would be collected before the CAT comes
into play. A report from the upstream sensor would include information that
the fuel mixture was 1.) too rich, 2.) too lean, or 3.) out of range,
without an indication as to the content of the mixture. The downstream
sensor does not know about to rich or too lean, it only knows that what
comes out of the CAT has not been cleaned up, relative to what went in. The
upstream sensor knows what goes into the CAT, the downstream sensor knows
what comes out.

Jeff Strickland

"DougW" <post.replies@invalid.address> wrote in message
news:TJfrh.41193$oA1.18948@newsfe19.lga...
> DougW wrote:
>
>> I've seen O2 sensors go bad, but not bad enough to set a fault. Usually
>> it's the forward one and the only thing it does is give you
>> bad milage and black exhaust. Figured the rearward one could go
>> equally bad and throw a code even if the cat was good.
>
> Actually that just made me think... if the forward O2 sensor
> is weak, the rear O2 sensor may very well call out a bad cat
> because the engine will be running rich.
>
> Too bad there isn't an easy way to test O2 sensors.


but, the upstream sensor will report a problem with the exhaust stream
before it enters the CAT, it will not cause the downstream sensor to report
that the CAT has failed.

I suppose the downstream _could_ report a failed CAT, but my instinct and
expereince say that the upstream sensor will report its own problem before
the downstream sensor will report a bad CAT.

Jeff Strickland

"DougW" <post.replies@invalid.address> wrote in message
news:TJfrh.41193$oA1.18948@newsfe19.lga...
> DougW wrote:
>
>> I've seen O2 sensors go bad, but not bad enough to set a fault. Usually
>> it's the forward one and the only thing it does is give you
>> bad milage and black exhaust. Figured the rearward one could go
>> equally bad and throw a code even if the cat was good.
>
> Actually that just made me think... if the forward O2 sensor
> is weak, the rear O2 sensor may very well call out a bad cat
> because the engine will be running rich.
>
> Too bad there isn't an easy way to test O2 sensors.


but, the upstream sensor will report a problem with the exhaust stream
before it enters the CAT, it will not cause the downstream sensor to report
that the CAT has failed.

I suppose the downstream _could_ report a failed CAT, but my instinct and
expereince say that the upstream sensor will report its own problem before
the downstream sensor will report a bad CAT.

Jeff Strickland

"DougW" <post.replies@invalid.address> wrote in message
news:TJfrh.41193$oA1.18948@newsfe19.lga...
> DougW wrote:
>
>> I've seen O2 sensors go bad, but not bad enough to set a fault. Usually
>> it's the forward one and the only thing it does is give you
>> bad milage and black exhaust. Figured the rearward one could go
>> equally bad and throw a code even if the cat was good.
>
> Actually that just made me think... if the forward O2 sensor
> is weak, the rear O2 sensor may very well call out a bad cat
> because the engine will be running rich.
>
> Too bad there isn't an easy way to test O2 sensors.


but, the upstream sensor will report a problem with the exhaust stream
before it enters the CAT, it will not cause the downstream sensor to report
that the CAT has failed.

I suppose the downstream _could_ report a failed CAT, but my instinct and
expereince say that the upstream sensor will report its own problem before
the downstream sensor will report a bad CAT.

Jeff Strickland

"DougW" <post.replies@invalid.address> wrote in message
news:TJfrh.41193$oA1.18948@newsfe19.lga...
> DougW wrote:
>
>> I've seen O2 sensors go bad, but not bad enough to set a fault. Usually
>> it's the forward one and the only thing it does is give you
>> bad milage and black exhaust. Figured the rearward one could go
>> equally bad and throw a code even if the cat was good.
>
> Actually that just made me think... if the forward O2 sensor
> is weak, the rear O2 sensor may very well call out a bad cat
> because the engine will be running rich.
>
> Too bad there isn't an easy way to test O2 sensors.


but, the upstream sensor will report a problem with the exhaust stream
before it enters the CAT, it will not cause the downstream sensor to report
that the CAT has failed.

I suppose the downstream _could_ report a failed CAT, but my instinct and
expereince say that the upstream sensor will report its own problem before
the downstream sensor will report a bad CAT.

Jeff Strickland

"bllsht" <nospam@dot.net> wrote in message
news:nigrq2h2vlm58ducoftq5of5rvi6khcsp9@4ax.com...
> On Tue, 16 Jan 2007 20:03:42 GMT, "Jeff Strickland"
> <crwlr@verizon.net> wrote:
>
>>
>>"bllsht" <nospam@dot.net> wrote in message
>>news:567mq2lcmmf2td23fj2mo0bsfndgffg805@4ax.com. ..
>>> On Sun, 14 Jan 2007 14:41:18 -0600, "DougW"
>>> <post.replies@invalid.address> wrote:
>>>
>>>>bspear78 wrote:
>>>>> I have a check engine light on, and the code is reading a 12 and a 72.
>>>>> I know what the 12 means, but could not find any explanation for a
>>>>> reading of 72. Anyone know what this means?
>>>>
>>>>72 Catalyst efficiency below required level. (Same as code 64)
>>>>
>>>>This means one of two things.
>>>>1) your catalytic converter is plugging/cracking/failing
>>>>2) the O2 sensor behind the cat is failing
>>>
>>> If the downstream O2 was failing you'd get a downstream O2 fault, not
>>> a cat efficiency fault.
>>>
>>
>>I'm not sure that is true in an OBD I car. The CAT is not a likely source
>>of
>>problems, and my money is on the after-CAT sensor going bad.
>
> An OBD I jeep wouldn't even have a downstream O2 sensor. You just lost
> your money.
>
>>
>>I am pretty sure that the downstream sensor can give an error that says
>>the
>>CAT is failing. This can happen because the system doesn't know that the
>>sensor is bad, it only knows that the input from the sensor is not
>>correct.
>>The sensor can fail in a mode that makes the system say that the CAT is
>>not
>>working when the reality is that the sensor itself has taken a bye.
>
> Wrong. A failed downstream sensor will not set a cat efficiency fault.
> In fact, the poorer the downstream sensor functions, the less likely a
> cat fault would result.
>
>>
>>Having said all of that, the OP says he has a 97 YJ. In the 97, it was
>>called a TJ, and in 97, the codes are OBD II.
>
> A 1997 vehicle would be OBD II, no matter what it was called, or who
> made it.
>

Well, then we're all chasing our shadows. Because, an OBD II car won't have
a Code 72 ...





>>
>>
>>
>>
>>

Jeff Strickland

"bllsht" <nospam@dot.net> wrote in message
news:nigrq2h2vlm58ducoftq5of5rvi6khcsp9@4ax.com...
> On Tue, 16 Jan 2007 20:03:42 GMT, "Jeff Strickland"
> <crwlr@verizon.net> wrote:
>
>>
>>"bllsht" <nospam@dot.net> wrote in message
>>news:567mq2lcmmf2td23fj2mo0bsfndgffg805@4ax.com. ..
>>> On Sun, 14 Jan 2007 14:41:18 -0600, "DougW"
>>> <post.replies@invalid.address> wrote:
>>>
>>>>bspear78 wrote:
>>>>> I have a check engine light on, and the code is reading a 12 and a 72.
>>>>> I know what the 12 means, but could not find any explanation for a
>>>>> reading of 72. Anyone know what this means?
>>>>
>>>>72 Catalyst efficiency below required level. (Same as code 64)
>>>>
>>>>This means one of two things.
>>>>1) your catalytic converter is plugging/cracking/failing
>>>>2) the O2 sensor behind the cat is failing
>>>
>>> If the downstream O2 was failing you'd get a downstream O2 fault, not
>>> a cat efficiency fault.
>>>
>>
>>I'm not sure that is true in an OBD I car. The CAT is not a likely source
>>of
>>problems, and my money is on the after-CAT sensor going bad.
>
> An OBD I jeep wouldn't even have a downstream O2 sensor. You just lost
> your money.
>
>>
>>I am pretty sure that the downstream sensor can give an error that says
>>the
>>CAT is failing. This can happen because the system doesn't know that the
>>sensor is bad, it only knows that the input from the sensor is not
>>correct.
>>The sensor can fail in a mode that makes the system say that the CAT is
>>not
>>working when the reality is that the sensor itself has taken a bye.
>
> Wrong. A failed downstream sensor will not set a cat efficiency fault.
> In fact, the poorer the downstream sensor functions, the less likely a
> cat fault would result.
>
>>
>>Having said all of that, the OP says he has a 97 YJ. In the 97, it was
>>called a TJ, and in 97, the codes are OBD II.
>
> A 1997 vehicle would be OBD II, no matter what it was called, or who
> made it.
>

Well, then we're all chasing our shadows. Because, an OBD II car won't have
a Code 72 ...





>>
>>
>>
>>
>>

Jeff Strickland

"bllsht" <nospam@dot.net> wrote in message
news:nigrq2h2vlm58ducoftq5of5rvi6khcsp9@4ax.com...
> On Tue, 16 Jan 2007 20:03:42 GMT, "Jeff Strickland"
> <crwlr@verizon.net> wrote:
>
>>
>>"bllsht" <nospam@dot.net> wrote in message
>>news:567mq2lcmmf2td23fj2mo0bsfndgffg805@4ax.com. ..
>>> On Sun, 14 Jan 2007 14:41:18 -0600, "DougW"
>>> <post.replies@invalid.address> wrote:
>>>
>>>>bspear78 wrote:
>>>>> I have a check engine light on, and the code is reading a 12 and a 72.
>>>>> I know what the 12 means, but could not find any explanation for a
>>>>> reading of 72. Anyone know what this means?
>>>>
>>>>72 Catalyst efficiency below required level. (Same as code 64)
>>>>
>>>>This means one of two things.
>>>>1) your catalytic converter is plugging/cracking/failing
>>>>2) the O2 sensor behind the cat is failing
>>>
>>> If the downstream O2 was failing you'd get a downstream O2 fault, not
>>> a cat efficiency fault.
>>>
>>
>>I'm not sure that is true in an OBD I car. The CAT is not a likely source
>>of
>>problems, and my money is on the after-CAT sensor going bad.
>
> An OBD I jeep wouldn't even have a downstream O2 sensor. You just lost
> your money.
>
>>
>>I am pretty sure that the downstream sensor can give an error that says
>>the
>>CAT is failing. This can happen because the system doesn't know that the
>>sensor is bad, it only knows that the input from the sensor is not
>>correct.
>>The sensor can fail in a mode that makes the system say that the CAT is
>>not
>>working when the reality is that the sensor itself has taken a bye.
>
> Wrong. A failed downstream sensor will not set a cat efficiency fault.
> In fact, the poorer the downstream sensor functions, the less likely a
> cat fault would result.
>
>>
>>Having said all of that, the OP says he has a 97 YJ. In the 97, it was
>>called a TJ, and in 97, the codes are OBD II.
>
> A 1997 vehicle would be OBD II, no matter what it was called, or who
> made it.
>

Well, then we're all chasing our shadows. Because, an OBD II car won't have
a Code 72 ...





>>
>>
>>
>>
>>