P0420 comes back after CAT and O2 sensors replaced

I don't think that is correct. I have cancelled the faulty code(the
same P0420) after replacement and after drive cycle is done, no faulty
code present(all monitors cleared). My car just passed State inspection
because of that. I feel frustrated why the faulty code comes back.

On Oct 11, 6:20 pm, "mrsteveo" wrote:
> After you replaced those parts, you likely need to reset your ECU.
> There are a couple ways to do it, or you may find it easier to
> disconnect the battery for a few hours then reconnect it.
>
> That will allow the ECU to get new parameters and adjust itself to the
> new parts.
>
>
>
> wrote:
> > Hi,
>
> > It is raining today, on my way home, the check light turned on and I
> > checked, it reads P0420. I just have CAT and 2 O2 sensors(front adn
> > back) replaced about a month, how come? what possibly went wrong? What
> > should I do next?- Hide quoted text -- Show quoted text -
 >> Stay informed about: P0420 comes back after CAT and O2 sensors replaced 

I purchased both O2 sensors from a Toyota Dealer.

On Oct 11, 6:46 pm, "Anthony Fremont" wrote:
> wrote:
> > I don't think that is correct. I have cancelled the faulty code(the
> > same P0420) after replacement and after drive cycle is done, no faulty
> > code present(all monitors cleared). My car just passed State
> > inspection because of that. I feel frustrated why the faulty code
> > comes back.This might interest you:http://www.aa1car.com/library/p0420_dtc.htm
>
> Perhaps the 02 sensor after the cat is to blame.
 >> Stay informed about: P0420 comes back after CAT and O2 sensors replaced 

Thanks Ray. I live in PA and I don't think that any place in US sells
leaded fuel
and I see no smoke coming out of exhaust pipe. The place where I have
CAT
replaced doesn't have scan tool. So I may have to send my car to other
place for the
scan in order to make sure that O2 sensors are good before exaust shop
can carry
out warranty otherwise, they probably won't do anything.

On Oct 11, 6:55 pm, "Ray O" <rokigawaATtristarassociatesDOTcom> wrote:
> wrote in messagenews:1160599775.545655.124910@k70g2000cwa.googlegroups.com...
>
> > Hi,
>
> > It is raining today, on my way home, the check light turned on and I
> > checked, it reads P0420. I just have CAT and 2 O2 sensors(front adn
> > back) replaced about a month, how come? what possibly went wrong? What
> > should I do next?My guess would be that the replacement catalytic converter is defective or
> something is contaminating the catalyst.
>
> Leaded fuel or huge amounts of engine oil leaking into the combustion
> chamber (with accompanying blue-gray clouds out the exhaust) could
> contaminate the catalyst, but if you don't see the blue-gray smoke and you
> haven't used leaded fuel, then a defective catalytic converter is likely.
>
> I would bring the car back to the place that installed the cat and sensors
> for a warranty repair.
>
> Good luck!
> --
>
> Ray O
> (correct punctuation to reply)
 >> Stay informed about: P0420 comes back after CAT and O2 sensors replaced 

wrote:
> I don't think that is correct. I have cancelled the faulty code(the
> same P0420) after replacement and after drive cycle is done, no faulty
> code present(all monitors cleared). My car just passed State
> inspection because of that. I feel frustrated why the faulty code
> comes back.

This might interest you:
http://www.aa1car.com/library/p0420_dtc.htm

Perhaps the 02 sensor after the cat is to blame.
 >> Stay informed about: P0420 comes back after CAT and O2 sensors replaced 

wrote in message

> Hi,
>
> It is raining today, on my way home, the check light turned on and I
> checked, it reads P0420. I just have CAT and 2 O2 sensors(front adn
> back) replaced about a month, how come? what possibly went wrong? What
> should I do next?
>

My guess would be that the replacement catalytic converter is defective or
something is contaminating the catalyst.

Leaded fuel or huge amounts of engine oil leaking into the combustion
chamber (with accompanying blue-gray clouds out the exhaust) could
contaminate the catalyst, but if you don't see the blue-gray smoke and you
haven't used leaded fuel, then a defective catalytic converter is likely.

I would bring the car back to the place that installed the cat and sensors
for a warranty repair.

Good luck!
--

Ray O
(correct punctuation to reply)
 >> Stay informed about: P0420 comes back after CAT and O2 sensors replaced 

wrote:
> I purchased both O2 sensors from a Toyota Dealer.

Did you read all that material? It listed quite a few things to
consider.
 >> Stay informed about: P0420 comes back after CAT and O2 sensors replaced 

"Anthony Fremont" wrote in message

> wrote:
>> I don't think that is correct. I have cancelled the faulty code(the
>> same P0420) after replacement and after drive cycle is done, no faulty
>> code present(all monitors cleared). My car just passed State
>> inspection because of that. I feel frustrated why the faulty code
>> comes back.
>
> This might interest you:
> http://www.aa1car.com/library/p0420_dtc.htm
>
> Perhaps the 02 sensor after the cat is to blame.
>

A bad after-cat O2 sensor is unlikely to give a false P0420 code.

Think of the O2 sensor as those EKG sticky things and the signal from the O2
sensor and the EKG. During closed loop operation, the pre-cat sensor will
put out a signal like an EKG. The cat should clean things up so that the
signal from the post-cat sensor should be less lively. If the signal from
the post cat sensor is the same signal as the pre-cat sensor, (lively), then
the cat is dead.
--

Ray O
(correct punctuation to reply)


If the cat is doing its job, then the signal from a good post-cat O2 sensor
should be les
 >> Stay informed about: P0420 comes back after CAT and O2 sensors replaced 

"Anthony Fremont" wrote in message

> wrote:
>> I purchased both O2 sensors from a Toyota Dealer.
>
> Did you read all that material? It listed quite a few things to consider.
>

The OP replaced the cat, pre-cat O2 sensor (referred to as sensor #1), and
post-cat O2 sensor (referred to as sensor #2).

DTC P0420 will be set if the signal from sensor #2 is the same as sensor #1.
In other words, sensor #2's signal should be kind of flat instead of lively.
If the signal from sensor #2 is lively, then P0420 comes on. A bad O2
sensor is more likely to put out a weak signal than a lively signal so a
false P0420 due to bad sensor #2 is not lively.


--

Ray O
(correct punctuation to reply)
 >> Stay informed about: P0420 comes back after CAT and O2 sensors replaced 

wrote in message

> Thanks Ray. I live in PA and I don't think that any place in US sells
> leaded fuel
> and I see no smoke coming out of exhaust pipe. The place where I have
> CAT
> replaced doesn't have scan tool. So I may have to send my car to other
> place for the
> scan in order to make sure that O2 sensors are good before exaust shop
> can carry
> out warranty otherwise, they probably won't do anything.

No regular gas stations sell leaded fuel, and AFAIK, leaded fuel is not
available in the U.S., but I like to cover all bases when I think of it

No smoke out of the tail pipe is good!

If the place that replaced the cat does not have a scan tool, then how did
you get the code? If you have a personal scan tool, perhaps the shop that
replaced the can use it to check the codes. You can chcek O2 sensor
operation with a volt meter.

If the cat is aftermarket, I'd bet that the replacement cat is bad.

Good luck!
--

Ray O
(correct punctuation to reply)

>
> On Oct 11, 6:55 pm, "Ray O" <rokigawaATtristarassociatesDOTcom> wrote:
>> wrote in
>> messagenews:1160599775.545655.124910@k70g2000cwa.googlegroups.com...
>>
>> > Hi,
>>
>> > It is raining today, on my way home, the check light turned on and I
>> > checked, it reads P0420. I just have CAT and 2 O2 sensors(front adn
>> > back) replaced about a month, how come? what possibly went wrong? What
>> > should I do next?My guess would be that the replacement catalytic
>> > converter is defective or
>> something is contaminating the catalyst.
>>
>> Leaded fuel or huge amounts of engine oil leaking into the combustion
>> chamber (with accompanying blue-gray clouds out the exhaust) could
>> contaminate the catalyst, but if you don't see the blue-gray smoke and
>> you
>> haven't used leaded fuel, then a defective catalytic converter is likely.
>>
>> I would bring the car back to the place that installed the cat and
>> sensors
>> for a warranty repair.
>>
>> Good luck!
>> --
>>
>> Ray O
>> (correct punctuation to reply)
>
 >> Stay informed about: P0420 comes back after CAT and O2 sensors replaced 

The scan tool I mean is the equipment to scan CAT system including O2
signals not the OBDII reader. I went to the local exhaust shop this
morning, they were not happy. They claimed that it was impossible for a
brand new CAT going bad within a month, there must have something wrong
with my car. They argued that there are a lot of factors which might
cause P0420 code set by the computer and they asked me to send my car
to dealer for check up, and they would not honor the warranty unless
dealer says it is bad(they were not convinced that CAT could be
defective). I am wondering whether there is other factor(s) that may
set P0420? I like to cancel the code and wait to see what will happen
next since it just passed the inspection and I have a year to find out.
I am wondering whether there may be false alarm for some unknown
reason. Anyway, it is very frustrated for me having to deal with P0420
code allover again.


On Oct 12, 12:39 am, "Ray O" <rokigawaATtristarassociatesDOTcom>
wrote:
> wrote in messagenews:1160609005.854927.323010@m7g2000cwm.googlegroups.com...
>
> > Thanks Ray. I live in PA and I don't think that any place in US sells
> > leaded fuel
> > and I see no smoke coming out of exhaust pipe. The place where I have
> > CAT
> > replaced doesn't have scan tool. So I may have to send my car to other
> > place for the
> > scan in order to make sure that O2 sensors are good before exaust shop
> > can carry
> > out warranty otherwise, they probably won't do anything.No regular gas stations sell leaded fuel, and AFAIK, leaded fuel is not
> available in the U.S., but I like to cover all bases when I think of it
>
> No smoke out of the tail pipe is good!
>
> If the place that replaced the cat does not have a scan tool, then how did
> you get the code? If you have a personal scan tool, perhaps the shop that
> replaced the can use it to check the codes. You can chcek O2 sensor
> operation with a volt meter.
>
> If the cat is aftermarket, I'd bet that the replacement cat is bad.
>
> Good luck!
> --
>
> Ray O
> (correct punctuation to reply)
>
>
>
>
>
> > On Oct 11, 6:55 pm, "Ray O" <rokigawaATtristarassociatesDOTcom> wrote:
> >> wrote in
> >> messagenews:1160599775.545655.124910@k70g2000cwa.googlegroups.com...
>
> >> > Hi,
>
> >> > It is raining today, on my way home, the check light turned on and I
> >> > checked, it reads P0420. I just have CAT and 2 O2 sensors(front adn
> >> > back) replaced about a month, how come? what possibly went wrong? What
> >> > should I do next?My guess would be that the replacement catalytic
> >> > converter is defective or
> >> something is contaminating the catalyst.
>
> >> Leaded fuel or huge amounts of engine oil leaking into the combustion
> >> chamber (with accompanying blue-gray clouds out the exhaust) could
> >> contaminate the catalyst, but if you don't see the blue-gray smoke and
> >> you
> >> haven't used leaded fuel, then a defective catalytic converter is likely.
>
> >> I would bring the car back to the place that installed the cat and
> >> sensors
> >> for a warranty repair.
>
> >> Good luck!
> >> --
>
> >> Ray O
> >> (correct punctuation to reply)- Hide quoted text -- Show quoted text -
 >> Stay informed about: P0420 comes back after CAT and O2 sensors replaced 

Thanks Ray. Dealer charges 3 times than the local shop and for my 2000
Corolla with 157K on it, it may not be worth to pay that much other
than buying a new one.
I will cancel the code first, it comes back again, I'll send it to
dealer for diagnosis.

I personally prefer dealership service departments to avoid the very
> predicament you are facing. Toyota requires dealerships to have certain
> equipment, among them an exhaust gas analyzer, their technicians are
> factory-trained, and the warranty is backed up by the most financially
> secure car company in the world.
>
> The logic of how the ECM sets Diagnostic Trouble Code (DTC) P0420 is such
> that the most likely cause is a bad catalytic converter. #1 and #2 O2
> sensors work the same way and detect the presence of oxygen (O2) in the
> exhaust.
>
> Sensor #1 is sniffing the exhaust gas before it passes through the cat, and
> with the injectors sending fuel in pulses, the amount of O2 in the exhaust
> gas varies, so the voltage signal from Sensor #1 varies up and down.
>
> Sensor #2 sniffs the exhaust gas after it has passed through the cat, and
> the cat is supposed to clean up the gas. If the cat is working, the the
> voltage signal from sensor #2 should be much more even than the signal from
> sensor #1. If the signal from sensor #2 is varying and looks the same as
> the signal from sensor #1, then the cat has not changed the exhaust and the
> ECM sets DTC P0420.
>
> Other possible causes of P0420 are a damaged exhaust manifold or exhaust
> pipe (you would probably hear an exhaust leak), engine coolant temperature
> sensor not working properly, retarded spark timing, or bad O2 sensor.
>
> At this point, your best bet is to have a dealership diagnose the condition
> and with a written diagnosis, go back to the shop that did the work if
> something they replaced is bad.
> --
>
> Ray O
> (correct punctuation to reply)
>
>
>
>
>
> > On Oct 12, 12:39 am, "Ray O" <rokigawaATtristarassociatesDOTcom>
> > wrote:
> >> wrote in
> >> messagenews:1160609005.854927.323010@m7g2000cwm.googlegroups.com...
>
> >> > Thanks Ray. I live in PA and I don't think that any place in US sells
> >> > leaded fuel
> >> > and I see no smoke coming out of exhaust pipe. The place where I have
> >> > CAT
> >> > replaced doesn't have scan tool. So I may have to send my car to other
> >> > place for the
> >> > scan in order to make sure that O2 sensors are good before exaust shop
> >> > can carry
> >> > out warranty otherwise, they probably won't do anything.No regular gas
> >> > stations sell leaded fuel, and AFAIK, leaded fuel is not
> >> available in the U.S., but I like to cover all bases when I think of it
> >>
>
> >> No smoke out of the tail pipe is good!
>
> >> If the place that replaced the cat does not have a scan tool, then how
> >> did
> >> you get the code? If you have a personal scan tool, perhaps the shop
> >> that
> >> replaced the can use it to check the codes. You can chcek O2 sensor
> >> operation with a volt meter.
>
> >> If the cat is aftermarket, I'd bet that the replacement cat is bad.
>
> >> Good luck!
> >> --
>
> >> Ray O
> >> (correct punctuation to reply)
>
> >> > On Oct 11, 6:55 pm, "Ray O" <rokigawaATtristarassociatesDOTcom> wrote:
> >> >> wrote in
> >> >> messagenews:1160599775.545655.124910@k70g2000cwa.googlegroups.com...
>
> >> >> > Hi,
>
> >> >> > It is raining today, on my way home, the check light turned on and I
> >> >> > checked, it reads P0420. I just have CAT and 2 O2 sensors(front adn
> >> >> > back) replaced about a month, how come? what possibly went wrong?
> >> >> > What
> >> >> > should I do next?My guess would be that the replacement catalytic
> >> >> > converter is defective or
> >> >> something is contaminating the catalyst.
>
> >> >> Leaded fuel or huge amounts of engine oil leaking into the combustion
> >> >> chamber (with accompanying blue-gray clouds out the exhaust) could
> >> >> contaminate the catalyst, but if you don't see the blue-gray smoke and
> >> >> you
> >> >> haven't used leaded fuel, then a defective catalytic converter is
> >> >> likely.
>
> >> >> I would bring the car back to the place that installed the cat and
> >> >> sensors
> >> >> for a warranty repair.
>
> >> >> Good luck!
> >> >> --
>
> >> >> Ray O
> >> >> (correct punctuation to reply)- Hide quoted text -- Show quoted text -- Hide quoted text -- Show quoted text -
 >> Stay informed about: P0420 comes back after CAT and O2 sensors replaced 

wrote in message

> The scan tool I mean is the equipment to scan CAT system including O2
> signals not the OBDII reader. I went to the local exhaust shop this
> morning, they were not happy. They claimed that it was impossible for a
> brand new CAT going bad within a month, there must have something wrong
> with my car. They argued that there are a lot of factors which might
> cause P0420 code set by the computer and they asked me to send my car
> to dealer for check up, and they would not honor the warranty unless
> dealer says it is bad(they were not convinced that CAT could be
> defective). I am wondering whether there is other factor(s) that may
> set P0420? I like to cancel the code and wait to see what will happen
> next since it just passed the inspection and I have a year to find out.
> I am wondering whether there may be false alarm for some unknown
> reason. Anyway, it is very frustrated for me having to deal with P0420
> code allover again.

I personally prefer dealership service departments to avoid the very
predicament you are facing. Toyota requires dealerships to have certain
equipment, among them an exhaust gas analyzer, their technicians are
factory-trained, and the warranty is backed up by the most financially
secure car company in the world.

The logic of how the ECM sets Diagnostic Trouble Code (DTC) P0420 is such
that the most likely cause is a bad catalytic converter. #1 and #2 O2
sensors work the same way and detect the presence of oxygen (O2) in the
exhaust.

Sensor #1 is sniffing the exhaust gas before it passes through the cat, and
with the injectors sending fuel in pulses, the amount of O2 in the exhaust
gas varies, so the voltage signal from Sensor #1 varies up and down.

Sensor #2 sniffs the exhaust gas after it has passed through the cat, and
the cat is supposed to clean up the gas. If the cat is working, the the
voltage signal from sensor #2 should be much more even than the signal from
sensor #1. If the signal from sensor #2 is varying and looks the same as
the signal from sensor #1, then the cat has not changed the exhaust and the
ECM sets DTC P0420.

Other possible causes of P0420 are a damaged exhaust manifold or exhaust
pipe (you would probably hear an exhaust leak), engine coolant temperature
sensor not working properly, retarded spark timing, or bad O2 sensor.

At this point, your best bet is to have a dealership diagnose the condition
and with a written diagnosis, go back to the shop that did the work if
something they replaced is bad.
--

Ray O
(correct punctuation to reply)
>
>
> On Oct 12, 12:39 am, "Ray O" <rokigawaATtristarassociatesDOTcom>
> wrote:
>> wrote in
>> messagenews:1160609005.854927.323010@m7g2000cwm.googlegroups.com...
>>
>> > Thanks Ray. I live in PA and I don't think that any place in US sells
>> > leaded fuel
>> > and I see no smoke coming out of exhaust pipe. The place where I have
>> > CAT
>> > replaced doesn't have scan tool. So I may have to send my car to other
>> > place for the
>> > scan in order to make sure that O2 sensors are good before exaust shop
>> > can carry
>> > out warranty otherwise, they probably won't do anything.No regular gas
>> > stations sell leaded fuel, and AFAIK, leaded fuel is not
>> available in the U.S., but I like to cover all bases when I think of it
>>
>>
>> No smoke out of the tail pipe is good!
>>
>> If the place that replaced the cat does not have a scan tool, then how
>> did
>> you get the code? If you have a personal scan tool, perhaps the shop
>> that
>> replaced the can use it to check the codes. You can chcek O2 sensor
>> operation with a volt meter.
>>
>> If the cat is aftermarket, I'd bet that the replacement cat is bad.
>>
>> Good luck!
>> --
>>
>> Ray O
>> (correct punctuation to reply)
>>
>>
>>
>>
>>
>> > On Oct 11, 6:55 pm, "Ray O" <rokigawaATtristarassociatesDOTcom> wrote:
>> >> wrote in
>> >> messagenews:1160599775.545655.124910@k70g2000cwa.googlegroups.com...
>>
>> >> > Hi,
>>
>> >> > It is raining today, on my way home, the check light turned on and I
>> >> > checked, it reads P0420. I just have CAT and 2 O2 sensors(front adn
>> >> > back) replaced about a month, how come? what possibly went wrong?
>> >> > What
>> >> > should I do next?My guess would be that the replacement catalytic
>> >> > converter is defective or
>> >> something is contaminating the catalyst.
>>
>> >> Leaded fuel or huge amounts of engine oil leaking into the combustion
>> >> chamber (with accompanying blue-gray clouds out the exhaust) could
>> >> contaminate the catalyst, but if you don't see the blue-gray smoke and
>> >> you
>> >> haven't used leaded fuel, then a defective catalytic converter is
>> >> likely.
>>
>> >> I would bring the car back to the place that installed the cat and
>> >> sensors
>> >> for a warranty repair.
>>
>> >> Good luck!
>> >> --
>>
>> >> Ray O
>> >> (correct punctuation to reply)- Hide quoted text -- Show quoted text -
>
 >> Stay informed about: P0420 comes back after CAT and O2 sensors replaced 

wrote:.

Ray wrote:

> > I personally prefer dealership service departments to avoid the very
> > predicament you are facing. Toyota requires dealerships to have certain
> > equipment, among them an exhaust gas analyzer, their technicians are
> > factory-trained, and the warranty is backed up by the most financially
> > secure car company in the world.

> Thanks Ray. Dealer charges 3 times than the local shop and for my 2000
> Corolla with 157K on it, it may not be worth to pay that much other
> than buying a new one.

What would the cost have been to have the dealer just diagnose this
problem, compared to what you've paid for the O2 sensors and catalytic
converter?

In another forum, a person didn't want to pay a Ford dealer almost $180
to diagnose a transmission slippage problem, so he instead went to
AAMCO, which took apart his transmission (maybe without his permission)
and now wants almost $700 just to put it back together. A qualified,
honest mechanic, no matter how expensive, is always cheaper than any of
the alternatives.
 >> Stay informed about: P0420 comes back after CAT and O2 sensors replaced