Check engine light and oxygen sensors

[scjwonder]

I have a ’00 Legacy GT and changed both oxygen sensors after getting a
Check engine code P0420 (i think) for oxygen sensors. Within a week
the check engine light is back- same code. Why would new sensors, fore
and aft not keep that "check engine" light off?

 

[scjwonder]

I found my solution on the "net". Apparently if the oxygen reading
at the fore and aft sensors is the same, it means that the converter
is not efficiently cleaning the exhaust fumes and likely needs to be
replaced, resulting in the check engine light.
Replacing the sensors probably wasn’t necessary in the first place.
Wish I’d known that B-4 I did it

 

[Dave]

I have a '00 Legacy GT and changed both oxygen sensors after getting a
Check engine code P0420 (i think) for oxygen sensors. Within a week
the check engine light is back- same code. Why would new sensors, fore
and aft not keep that "check engine" light off?

--
Posted at author's request, using moderated http://www.AutoBoardz.com
interface
Thread archive:
http://www.AutoBoardz.com/Check-engine-light-oxygen-sensors-ftopict247877.html

It's because the PO420 code doesn't have anything to do with the O2 sensors
per se. That code sets when the sensors determine through correlation that
the catalytic converter is not doing it's job. If the O2 sensors themselves
have a problem it sets a completely different code(s).

 

[Dave]

I found my solution on the "net". Apparently if the oxygen reading
at the fore and aft sensors is the same, it means that the converter
is not efficiently cleaning the exhaust fumes and likely needs to be
replaced, resulting in the check engine light.
Replacing the sensors probably wasn't necessary in the first place.
Wish I'd known that B-4 I did it.

Yeah, I tried to explain that in an earlier thread on this ng, but folks see
that $1400 tab for a set of converters and their eyes glaze over. Suddenly
the cheaper O2 sensors look a lot better. Oh well look on the bright side.
If you haven't changed the O2 sensors before, you were due for them anyway.

 

[me]

It's because the PO420 code doesn't have anything to do with the O2 sensors
per se. That code sets when the sensors determine through correlation that
the catalytic converter is not doing it's job. If the O2 sensors themselves
have a problem it sets a completely different code(s).

what is the code if it is the O2 sensors themselves?

 

[Dave]

what is the code if it is the O2 sensors themselves?

There are dozens. Here is the largest group...

P0130 O2 Circuit (Bank 1, Sensor 1)
P0131 O2 Circuit Low Voltage (Bank 1, Sensor 1)
P0132 O2 Circuit High Voltage (Bank 1, Sensor 1)
P0133 O2 Circuit Slow Response (Bank 1, Sensor 1)
P0134 O2 Circuit No Activity Detected (Bank 1, Sensor 1)
P0135 O2 Heater Circuit (Bank 1, Sensor 1)
P0136 O2 Circuit (Bank 1, Sensor 2) (See P2270, P2271)
P0137 O2 Circuit Low Voltage (Bank 1, Sensor 2)
P0138 O2 Circuit High Voltage (Bank 1, Sensor 2)
P0139 O2 Circuit Slow Response (Bank 1, Sensor 2)
P013A O2 Sensor Slow Response - Rich to Lean (Bank 1, Sensor 2)
P013B O2 Sensor Slow Response - Lean to Rich (Bank 1, Sensor 2)
P013C O2 Sensor Slow Response - Rich to Lean (Bank 2, Sensor 2)
P013D O2 Sensor Slow Response - Lean to Rich (Bank 2, Sensor 2)
P0140 O2 Circuit No Activity Detected (Bank 1, Sensor 2)
P0141 O2 Heater Circuit (Bank 1, Sensor 2)
P0142 O2 Circuit (Bank 1, Sensor 3)
P0143 O2 Circuit Low Voltage (Bank 1, Sensor 3)
P0144 O2 Circuit High Voltage (Bank 1, Sensor 3)
P0145 O2 Circuit Slow Response (Bank 1, Sensor 3)
P0146 O2 Circuit No Activity Detected (Bank 1, Sensor 3)
P0147 O2 Heater Circuit (Bank 1, Sensor 3)
P0150 O2 Circuit (Bank 2, Sensor 1)
P0151 O2 Circuit Low Voltage (Bank 2, Sensor 1)
P0152 O2 Circuit High Voltage (Bank 2, Sensor 1)
P0153 O2 Circuit Slow Response (Bank 2, Sensor 1)
P0154 O2 Circuit No Activity Detected (Bank 2, Sensor 1)
P0155 O2 Heater Circuit (Bank 2, Sensor 1)
P0156 O2 Circuit (Bank 2, Sensor 2) (See P2272, P2273)
P0157 O2 Circuit Low Voltage (Bank 2, Sensor 2)
P0158 O2 Circuit High Voltage (Bank 2, Sensor 2)
P0159 O2 Circuit Slow Response (Bank 2, Sensor 2)
P0160 O2 Circuit No Activity Detected (Bank 2, Sensor 2)
P0161 O2 Heater Circuit (Bank 2, Sensor 2)
P0162 O2 Circuit (Bank 2, Sensor 3)
P0163 O2 Circuit Low Voltage (Bank 2, Sensor 3)
P0164 O2 Circuit High Voltage (Bank 2, Sensor 3)
P0165 O2 Circuit Slow Response (Bank 2, Sensor 3)
P0166 O2 Circuit No Activity Detected (Bank 2, Sensor 3)
P0167 O2 Heater Circuit (Bank 2, Sensor 3)

And here is the group of cat codes...

P0420 Catalyst System Efficiency Below Threshold (Bank 1)
P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)
P0422 Main Catalyst Efficiency Below Threshold (Bank 1)
P0423 Heated Catalyst Efficiency Below Threshold (Bank 1)
P0424 Heated Catalyst Temperature Below Threshold (Bank 1)
P0425 Catalyst Temperature Sensor Circuit (Bank 1, Sensor Circuit 1)
P0426 Catalyst Temperature Sensor Circuit Range/Performance (Bank 1, Sensor
Circuit 1)
P0427 Catalyst Temperature Sensor Circuit Low (Bank 1, Sensor Circuit 1)
P0428 Catalyst Temperature Sensor Circuit High (Bank 1, Sensor Circuit 1)
P0429 Catalyst Heater Control Circuit (Bank 1)
P042A Catalyst Temperature Sensor Circuit (Bank 1, Sensor Circuit 2)
P042B Catalyst Temperature Sensor Circuit Range/Performance (Bank1, Sensor
Circuit 2)
P042C Catalyst Temperature Sensor Circuit Low (Bank 1, Sensor Circuit 2)
P042D Catalyst Temperature Sensor Circuit High (Bank 1, Sensor Circuit 2)

 

[kaplan3jiim]

Well in my case first the Cats were replaced, now one of the 02
sensors and within a couple hundred miles I have the CEL once again.
$1500+ poorer.

Won't be spending any more money on this nonsense.

But I do have to pass an emissions test next June. After I have the
codes cleared, how long does the car have to be driven to have the
computer have the proper status to pass an emission test. I'm
assuming there might be a window of opportunity before the CEL light
returns.

 

[.._..]

The window of opportunity is often only a few seconds. It will be on when
the car thinks its warm.

And, what's your problem with running a clean car? It seems to me like you
need a new cat, so get a new cat. Or get a new fucking car (which will have
a new cat).

If you are looking for excuses or a way to cheat the systems put in place to
prevent asstards like you from fucking up my ability to breathe, maybe you
should consider fucking off. Go back to mexico you cheap ass ****** piece
of shit.

 

[Todd H.]

Well in my case first the Cats were replaced, now one of the 02
sensors and within a couple hundred miles I have the CEL once again.
$1500+ poorer.

Won't be spending any more money on this nonsense.

But I do have to pass an emissions test next June. After I have the
codes cleared, how long does the car have to be driven to have the
computer have the proper status to pass an emission test. I'm
assuming there might be a window of opportunity before the CEL light
returns.

It seems to vary. Mine's getting shorter.

Once you clear teh code, it resets counters that take time to become
valid. So, don't think you can clear the code and then go have your
emissions tested -- you won't pass if the sensors/tests that require a
longer time period to become "ok" aren't yet in that state.

It seemed like you needed about oh... 25 miles or so after clearing
codes to get clear. When I first started fighting p0420 that time
window of being clear before anothe rcel would set was about a
month.

Now, the window is getting smaller or non-existent. I need a new
cat, myself, apparently. I certainly don't want to be poluting but I
also don't want to be throwing money on what might be a false
indication of a problem.

 

[kaplan3jiim]

The window of opportunity is often only a few seconds. It will be on when
the car thinks its warm.

And, what's your problem with running a clean car? It seems to me like you
need a new cat, so get a new cat. Or get a new fucking car (which will have
a new cat).

If you are looking for excuses or a way to cheat the systems put in place to
prevent asstards like you from fucking up my ability to breathe, maybe you
should consider fucking off. Go back to mexico you cheap ass ****** piece
of shit.

Probably shouldn't reply to an idiot who can't read but if you could
you'd noticed I have spent $1500 to replace both catalytic converters
and 02 sensors in the last few months and the same code keeps coming
back and I seriously doubt that there is truly an emission issue at
this point.

 

[kaplan3jiim]

It seems to vary. Mine's getting shorter.

Once you clear teh code, it resets counters that take time to become
valid. So, don't think you can clear the code and then go have your
emissions tested -- you won't pass if the sensors/tests that require a
longer time period to become "ok" aren't yet in that state.

It seemed like you needed about oh... 25 miles or so after clearing
codes to get clear. When I first started fighting p0420 that time
window of being clear before anothe rcel would set was about a
month.

Now, the window is getting smaller or non-existent. I need a new
cat, myself, apparently. I certainly don't want to be poluting but I
also don't want to be throwing money on what might be a false
indication of a problem.

Actually, without being precise, it does seem that the window until
the CEL returns has gotten shorter with each trip to the repair shop.
I did know it takes time to become valid. It seems that the repair
shop had told me they were driving it long enough to return it to a
valid state so if that's the case I'll run right down to emission
station from the repair shop (in Joliet, IL)

 

[jim]

take a pill, man.

The window of opportunity is often only a few seconds. It will be on when
the car thinks its warm.

And, what's your problem with running a clean car? It seems to me like you
need a new cat, so get a new cat. Or get a new fucking car (which will have
a new cat).

If you are looking for excuses or a way to cheat the systems put in place to
prevent asstards like you from fucking up my ability to breathe, maybe you
should consider fucking off. Go back to mexico you cheap ass ****** piece
of shit.

 

[scjwonder]

After I replaced the CATs it took about two weeks for the CEL the
first time and with each reset it has taken much less time. I was told
by the tech at MIDAS who installed my CATs that the CEL often comes
back if it is not a Subaru OEM replacement CAT, so now I just ignore
it

 

[kaplan3jiim]

The car is going back in Monday to ascertain whether the replacement
non-Subaru CATs are indeed defective.

Can't ignore it for more than 7 months as we have an emission
inspection which consists of reading the codes set for a pass/fail
test.

So was the tech saying it came on simply because of its origins (an
electrical signal issue?) or because the emitted exhaust was not up to
standards?

 

[tom klein]

pardon me, but that seems like a load of malarkey - either midas'
aftermarket cat works properly, or it should never have been used in
that application. and it doesn't say much about midas'
professionalism, either.

 

[kaplan3jiim]

The car now has 2 new Cats to replace the original replacements.
Hopefully this is the end of the journey, but only 100 miles so farf
since the work was done. Although they are non-OEM, they are suppose
to be OEM spec'd & do have a warranty of 5 years so I guess I can just
keep replacing them until one work sright if necesary. Sigh.

 

[tom klein]

I have a ’00 Legacy GT and changed both oxygen sensors after getting a
Check engine code P0420 (i think) for oxygen sensors. Within a week
the check engine light is back- same code. Why would new sensors, fore
and aft not keep that "check engine" light off?

here's a related but slightly different question: i do not get a cel,
but if i hook up a code reader, the "c" test (cat. converter) always
flashes, indicating that it didn't complete. this has been going on
for months (well over 100 "trips"), and with no (apparent) ill effects.
what does this mean?

'02 legacy wagon, 5mt, 168,000 km

 

[Carl 1 Lucky Texan]

here's a related but slightly different question: i do not get a cel,
but if i hook up a code reader, the "c" test (cat. converter) always
flashes, indicating that it didn't complete. this has been going on for
months (well over 100 "trips"), and with no (apparent) ill effects. what
does this mean?

'02 legacy wagon, 5mt, 168,000 km

Hmmmm....be interetsing to try that reader on a different soob and a
different reader on yours. Some (I think only very cheap) readers may
not read the ISO protocol correctly.

If it isn't the reader - maybe the ECU is bad?

Carl