Request for diagnostic advice to pinpoint least expensive solution to resolve persistent P0421 OBDII DTC

The goal is to learn how to diagnose a P0421 using an OBDII scanner.

Request for diagnostic advice to pinpoint least expensive solution
to resolve persistent P0421 OBDII DTC given...

1. College kid's early 2000s Mitsubishi fails OBD-only smog
2. Neighbor asks me to help out (as if I know anything, which I don't)
3. However, I can borrow the vehicle & run live tests via OBDII scanner

Question is, what do I look for in terms of "live data" to help diagnose?

DETAILS FOLLOW:
o 180K miles, circa 2003 or 2004 model year (I think)
o Service Engine Soon has been reported to be on for a year
o New pre cat 02 sensor had reportedly already been installed 3 months ago
o Lambda O2 Sensor always tests OK on the OBDII scanner
o CAT always tests OK on the OBDII scanner
o All other registers always test OK on the OBDII scanner
o The P0421 DTC is set "sometimes" when the CAT register is set
o I cleared the DTCs a few times but the P0421 came back each time
o I/M Readiness lasts only about a day or three before DTC occurs again
o Always the same DTC and always only that single DTC
o Warm Up Catalyst Efficiency Below Threshold Bank 1

The question isn't "Does it require a new cat", nor is the question
"What parts can we blindly throw at it until it finally works".

The question is what "live data" do we look for to _diagnose_
what the problem actually is (and what's the options for repair).

My main dilemma that I'm asking for your help on is what
do I look for in the "live data" when I take it for a test
drive with the OBD scanner?

BTW, I'm familiar with the concept of the Federal Test Procedure
(also known as the Drive Cycle), but I don't have the specific
drive cycle for this specific vehicle (but I can guess based on
the drive cycle for other vehicles, although not the same marquee).

PHOTOS FOLLOW:
o <https://i.postimg.cc/KzTmdCBx/obd01.jpg> Failed smog
o <https://i.postimg.cc/nrYpL73W/obd02.jpg> 180,000 miles
o <https://i.postimg.cc/65tBmj3x/obd03.jpg> 1 DTC with all registers set
o <https://i.postimg.cc/fRbDwVmH/obd04.jpg> P0421 below threshold, bank 1
o <https://i.postimg.cc/437G9j8V/obd05.jpg> Lambda sensor OK
o <https://i.postimg.cc/kgh9D7y7/obd06.jpg> CAT OK
o <https://i.postimg.cc/901KSB8c/obd08.jpg> EVAP, MIS, FUEL, OK
o <https://i.postimg.cc/26nfHhN5/obd07.jpg> Live data page 1
o <https://i.postimg.cc/v8vJcQQC/obd09.jpg> Live data page 2
o <https://i.postimg.cc/vHqCt3xz/obd10.jpg> Live data page 3
o <https://i.postimg.cc/NjKSJm1w/obd11.jpg> Live data page 4
o <https://i.postimg.cc/T3BSZ195/obd12.jpg> What values do I look for?

QUESTION:
o What do I look for in terms of "live data" to help diagnose?
--
The goal is to learn how to diagnose a P0421 using an OBDII scanner.

Arlen Holder > wrote:
>The goal is to learn how to diagnose a P0421 using an OBDII scanner.

Okay. You're getting an error which is saying that the ratio between the
forward O2 sensor and the rear O2 sensor is not what it should be.

And you are -not- getting an error that is saying that the forward O2
sensor is out of range, which is also important. The fact that you are
-not- getting that error tells you that the exhaust coming out of the
engine isn't THAT far off.

Now, it could still be off. It's still worth doing all the basic sanity
checks: change the plugs, make sure the cables are good and there are no
signs of arcing, make sure fuel pressure is good and that the exhaust smells
okay. The idle RPM should be right on the mark and should not be drifting.

If you're spitting unburnt fuel down the tailpipe, the converter is going
to be unhappy.

Now, look at the data for the two O2 sensors and look at what happens when
the engine starts up. The first sensor should climb up into the normal range
once it warms up. The second sensor isn't going to get into the normal
range until the converter warms up. The shape of the time-domain curves
will be different from one model to another, but just watch. The second O2
sensor might be bad... the converter might be bad..... and it's -possible-
that the converter is just overwhelmed with junk (although that it is least
likely of the three since you aren't seeing any other errors). But watch
the numbers.

And you want to look at it for as long as you can... because you can have a
loose O2 sensor cable, or a bad engine ground, and see weird spikes on those
graphs that occur very very seldom but which can lead the computer to set
an error.

If this engine has two banks, you can compare the two and that is very very
useful information. Since you don't give the engine or model it's hard to
tell if that will help you or not though.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On 17 Nov 2020 23:57:22 -0000, Scott Dorsey wrote:

> Okay. You're getting an error which is saying that the ratio between the
> forward O2 sensor and the rear O2 sensor is not what it should be.

Hi Scott,
I appreciate your advice as I'm not quite sure what readings to look at.
o Let me know what information you need to help me figure out what to do.

OK. That's a good start as I was unaware that was what it tells us.
o The main problem is what should that front-to-back ratio be?
And under what conditions?

> And you are -not- getting an error that is saying that the forward O2
> sensor is out of range, which is also important.

It's a 3-month old forward sensor.
o I have no idea about the age of the rear sensor though.

In the daylight, if it's dry outside, I will ask to look under the vehicle.
o There is only one bank (AFAIK).

> The fact that you are
> -not- getting that error tells you that the exhaust coming out of the
> engine isn't THAT far off.

From what I've gathered, I agree.
It goes hours, sometimes days between the registers being set and the DTC.
However I watched the dash light go on the instant the CAT register set.
So it's "very" close, I think.

> Now, it could still be off. It's still worth doing all the basic sanity
> checks: change the plugs, make sure the cables are good and there are no
> signs of arcing, make sure fuel pressure is good and that the exhaust smells
> okay. The idle RPM should be right on the mark and should not be drifting.

Note there is no evidence whatsoever of a misfire. Nor of rough idle.
No smoke. No smells.
But it's still good to replace the plugs (but that's throwing parts at it).

> If you're spitting unburnt fuel down the tailpipe, the converter is going
> to be unhappy.

Yup. The goal, always, is to not destroy the cat.

> Now, look at the data for the two O2 sensors and look at what happens when
> the engine starts up. The first sensor should climb up into the normal range
> once it warms up. The second sensor isn't going to get into the normal
> range until the converter warms up.

OK. Thanks. I'll look for that first.
a. Start cold (overnight)
b. Start & idle & watch the two O2 sensors (pre cat and post cat)
c. I'm looking for the pre-cat to get into range sooner than the post cat

Are these the two voltage readings for the two oxygen sensors?
o O2SLOC B1S12--B2S----
o O2B1S1(V) 0.120
o O2B1S2(V) 0.140
<https://i.postimg.cc/NjKSJm1w/obd11.jpg>

When do I take the readings once the engine is warmed up?

Here's a reading, for example, taken at a steady 50mph:
o O2SLOC B1S12--B2S----
o O2B1S1(V) 0.400
o O2B1S2(V) 0.160
<https://i.postimg.cc/T3BSZ195/obd12.jpg>

> The shape of the time-domain curves
> will be different from one model to another, but just watch. The second O2
> sensor might be bad... the converter might be bad..... and it's -possible-
> that the converter is just overwhelmed with junk (although that it is least
> likely of the three since you aren't seeing any other errors). But watch
> the numbers.
> And you want to look at it for as long as you can... because you can have a
> loose O2 sensor cable, or a bad engine ground, and see weird spikes on those
> graphs that occur very very seldom but which can lead the computer to set
> an error.

I don't have the necessary equipment to save or see graphs.
o I can only see the live data as it happens.

Here is a live shot of the data at a steady 60 mph:
o O2B1S1(V) 0.120
o O2B1S2(V) 0.180
<https://i.postimg.cc/qRLqQZVv/obd13.jpg>

And another stead 60 mph reading:
o O2B1S1(V) 0.740
o O2B1S2(V) 0.640
<https://i.postimg.cc/Gtm2fJdJ/obd14.jpg>

I'm not sure the speed for this one:
o O2B1S1(V) 0.460
o O2B1S2(V) 0.120
<https://i.postimg.cc/YqV9XWVY/obd15.jpg>

Here's another reading, at a steady 20 mph:
o O2B1S1(V) 0.700
o O2B1S2(V) 0.400
<https://i.postimg.cc/x1zdBdD7/obd16.jpg>

And another at an unrecorded speed:
o O2B1S1(V) 0.820
o O2B1S2(V) 0.600
<https://i.postimg.cc/3wQwhH3w/obd17.jpg>

And another at an unrecorded speed:
o O2B1S1(V) 0.500
o O2B1S2(V) 0.800
<https://i.postimg.cc/yNh8YrNy/obd18.jpg>

Here's one taken at 50 mph:
o O2B1S1(V) 0.140
o O2B1S2(V) 0.760
<https://i.postimg.cc/k4nXVF63/obd19.jpg>

Another, albeit in a sharp turn, at 50 mph:
o O2B1S1(V) 0.800
o O2B1S2(V) 0.780
<ttps://i.postimg.cc/nz0VqjBs/obd20.jpg>

Another, on the highway but at an unrecorded speed:
o O2B1S1(V) 0.780
o O2B1S2(V) 0.140
<https://i.postimg.cc/RhYMbkP4/obd21.jpg>

Taken shortly thereafter at 55 mph:
o O2B1S1(V) 0.820
o O2B1S2(V) 0.700
<https://i.postimg.cc/bYVNJnqW/obd22.jpg>

Another on the highway at an unrecorded speed (2500 RPM though):
o O2B1S1(V) 0.140
o O2B1S2(V) 0.220
<https://i.postimg.cc/kXVMt544/obd23.jpg>

Another on the highway at 60 mph:
o O2B1S1(V) 0.400
o O2B1S2(V) 0.160
<https://i.postimg.cc/J49r3WDQ/obd24.jpg>

Another on the highway at 60 mph moments later:
o O2B1S1(V) 0.620
o O2B1S2(V) 0.300
<https://i.postimg.cc/GmGcqSvn/obd25.jpg>

Another on the highway at 60 mph:
o O2B1S1(V) 0.120
o O2B1S2(V) 0.180
<https://i.postimg.cc/rp9qL0hh/obd26.jpg>

Another on the highway at 60 mph:
o O2B1S1(V) 0.440
o O2B1S2(V) 0.240
<https://i.postimg.cc/sXDsfjq3/obd27.jpg>

This one at 1,000 rpm (probably idling):
o O2B1S1(V) 0.220
o O2B1S2(V) 0.840
<https://i.postimg.cc/fL2ZV4Qm/obd28.jpg>

Still at 1000 rpm shortly thereafter:
o O2B1S1(V) 0.060
o O2B1S2(V) 0.760
<https://i.postimg.cc/mZnBRGwG/obd29.jpg>

Another at 1000 rpm:
o O2B1S1(V) 0.600
o O2B1S2(V) 0.480
<https://i.postimg.cc/43XTw4Jp/obd30.jpg>

Yet another at 1000 rpm:
o O2B1S1(V) 0.240
o O2B1S2(V) 0.140
<https://i.postimg.cc/tTTGQrnT/obd31.jpg>

Here's one at 10mph and 2300 rpm:
o O2B1S1(V) 0.820
o O2B1S2(V) 0.780
<https://i.postimg.cc/QCKLVHbZ/obd32.jpg>

I'm not sure if it's normal for these readings.
o I do wish I had a graph - but I only seem to have live data.

> If this engine has two banks, you can compare the two and that is very very
> useful information. Since you don't give the engine or model it's hard to
> tell if that will help you or not though.

Only one bank, thank God.

BTW, I just checked.
It's a 2004 Mitsubishi Lancer ES, 2.0liter SOHC 4G94 engine
--
I appreciate your advice as I'm not quite sure what readings to look at.

[snip]

Given that you're in a State that mandates emission inspections...

Step 1: use your scanner to _clear_ the error code.
If you're very lucky, it won't show back up.

Alternatively, if it shows up again every 500 miles,
just live with it [a].

If it comes right back after 30 nominal miles (that
is, once the system decides to do the rechecks), then
you've got to figure out what's wrong

In my case, with a different car and a slighrly different
OBD code, it turned out (we discovered after _lots_ of
misdirection) that the EGR was pouring plenty of garbage
into the exhaust, coating the catalytic converter, and
messing things up.

Once we replaced the EGR this went from getting the
error light/message 20 miles after reset to.. the
aforemention very roughly every 500 miles.

Presumption was/is that there's caked on gunk which
is interfering with the converter, but... it's not
enough to set it off right away.

And, alas in my case, no easy way to pull the
converter and clean it. (That's an option
with some other cars).

[a] So what I did was check the OBD readings
fifty miles before my last inspection [b], make
sure it was ok, head to the inspection station,
stop outside, double check the reading, and
bring it in.

[b] the OBD ssytem needs to monitor the car for
thirty or so miles after a reset, so I couldn't
just clear it at the curb.

For good measure, which might or might not have
made a difference but which made me feel better, I

1: filled the last couple of tanks with ethanol_free
gasoline (which was also higher octane)
and
2: poured in Luke Oil cleanser into the tank.

good luck

--
__________________________________________________ ___
Knowledge may be power, but communications is the key

[to foil spammers, my address has been double rot-13 encoded]

Arlen Holder > wrote:
>On 17 Nov 2020 23:57:22 -0000, Scott Dorsey wrote:
>
>> Okay. You're getting an error which is saying that the ratio between the
>> forward O2 sensor and the rear O2 sensor is not what it should be.
>
>I appreciate your advice as I'm not quite sure what readings to look at.
>o Let me know what information you need to help me figure out what to do.

Read the message over again. What you want to know is in the graphs for the
two O2 sensors and how they track.

Don't worry so much about the ratio itself, worry about how they track.

But you will not figure the problem out just by looking at the plots, you will
figure the problem out by thinking about how the emission control system works.

>When do I take the readings once the engine is warmed up?

All the time. You take them before it warms up, you take them after it warms
up, you take them over the course of driving around. What you are looking for
is trends and correlation more than specific values.

>> The shape of the time-domain curves
>> will be different from one model to another, but just watch. The second O2
>> sensor might be bad... the converter might be bad..... and it's -possible-
>> that the converter is just overwhelmed with junk (although that it is least
>> likely of the three since you aren't seeing any other errors). But watch
>> the numbers.
>> And you want to look at it for as long as you can... because you can have a
>> loose O2 sensor cable, or a bad engine ground, and see weird spikes on those
>> graphs that occur very very seldom but which can lead the computer to set
>> an error.
>
>I don't have the necessary equipment to save or see graphs.
>o I can only see the live data as it happens.

Well, that's useless. So you're going to have to plot it on semilog paper
by hand. Your scanner does not provide ANY plotting functions?
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On 18 Nov 2020 13:17:51 -0000, Scott Dorsey wrote:

> What you want to know is in the graphs for the
> two O2 sensors and how they track.

Hi Scott,

Thanks for that gentle reminder, as I will borrow the vehicle today, but I
have to check it in their garage, as I only get one chance a day to test
warmup conditions (I think, at least, at Silicon Valley temperatures).

> But you will not figure the problem out just by looking at the plots, you will
> figure the problem out by thinking about how the emission control system works.

Well, to tell it to you bluntly, I'm not omnipotent.
o I didn't know anything when I first posted this thread

I didn't even know the _range_, which I've since learned is basically about
~100 mV for lean conditions, to about ~900mv for rich conditions.

Nor did I know when I opened this thread, what the expected differences
were for the upstream versus downstream lambda sensors, where the front
should be changing far more frequently than the rear (which should read, as
much as possible, a stoichiometric ~450mv under steady warmed load).

What I didn't know at first was why voltages bounced all over the place.

But now I've boned up, a tiny bit, on the fact that the bouncing around of
the voltages is "normal", at least for the upstream side, since the engine
computer is constantly adjusting the ratio (within seconds) depending on a
whole bunch of factors, not the least of which is engine loading.

Further honing what I didn't realize at first is that there are two states:
1. Steady state (which is where most of my prior measurements lay), &
2. Warmup state (which is where I need to focus, as you had advised me).

What I need is what most sites don't provide, which is I need to learn more
about this warmup process of the cat, since it's a 'warmup efficiency' DTC.

I only get about a minute (or two at most), right?

>>When do I take the readings once the engine is warmed up?
>
> All the time. You take them before it warms up, you take them after it warms
> up, you take them over the course of driving around. What you are looking for
> is trends and correlation more than specific values.

Yes. But.

I belatedly realized I need to focus specifically on 'warmup efficiency'.
o My main focus now revolves around how to diagnose 'warmup efficiency'.

Obviously I need to figure out what 'warmup' even means, which I need to do
as my homework, since I'm not even sure how long warmup of the cat lasts
(the sensors only take ~20 seconds to warm up, but I'm not sure how long it
takes the cat to warm up, nor, if a cat can even warm up twice in the same
day given Silicon Valley typical warm nightly & daily temperature ranges).

I'm gonna borrow the vehicle today but it's wet outside so I'm not sure if
I'm gonna crawl underneath to check the rear lambda sensor; but I will at
least check the live OBD data during the first 20 seconds of the lambda
sensor warmup and then the first few minutes, where I'm guessing a cat
warms up in, oh, I don't know, within the first five minutes perhaps?
o I'll look at an engine temperature reading (coolant perhaps?)
o Then I'll look at the front & rear sensors (and what they do over time)

I may need to buy an infrared thermometer to check how the cat warms up in
the first five minutes with the vehicle on jack stands; but no sense in
doing that until I learn more about how the cat _should_ warm up, as the
numbers won't mean anything if they're measured under the wrong sequence.

>>I don't have the necessary equipment to save or see graphs.
>>o I can only see the live data as it happens.
>
> Well, that's useless. So you're going to have to plot it on semilog paper
> by hand. Your scanner does not provide ANY plotting functions?

My OBD scanner _does_ provide plotting functions; but it's second hand (I
picked it up at a garage sale for a few bucks). It works fine for live
data, but I don't have the cabling or software to view the graphs.
o Harbor Freight item #60693 Cen-Tech OBDII/EOBD Deluxe Scanner

It also does 'freeze frame' but I haven't even started to look at what that
means to me, in terms of how I can use that to diagnose warmup efficiency.

In summary, I started out with the wrong assumptions, and I ran the wrong
tests, and I didn't know how the system worked... but I'm reducing my
ignorance by reading up on how the system works, and I'm gonna focus on the
all-important warmup process, where I think I only get one chance a day.
--
Experience is knowing which measurements to take, when, where, & why.

On 11/17/2020 6:06 PM, Arlen Holder wrote:
> The goal is to learn how to diagnose a P0421 using an OBDII scanner.
>
> Request for diagnostic advice to pinpoint least expensive solution
> to resolve persistent P0421 OBDII DTC given...
>
> 1. College kid's early 2000s Mitsubishi fails OBD-only smog
> 2. Neighbor asks me to help out (as if I know anything, which I don't)
> 3. However, I can borrow the vehicle & run live tests via OBDII scanner
>
> Question is, what do I look for in terms of "live data" to help diagnose?
>
> DETAILS FOLLOW:
> o 180K miles, circa 2003 or 2004 model year (I think)
> o Service Engine Soon has been reported to be on for a year

This is easy to diagnose. You just sell that POS and get a newer one.
You know better, Arlen dear. Dumb kid won't graduate anyway. That is
why I married you.

For the record, here's a description of what the computer checks for:
<http://www.smogtechinstitute.com/smogtech/images/PDFs/OBDIIDataInterpretation.pdf>

1. The computer looks to see if the heated oxygen sensor voltage rises
above 600mV and falls below 300mV and switches in less than 100ms.

2. When the cat is working properly there is almost no switching action
from the rear oxygen sensor.

3. To test the rear oxygen sensor, the computer forces a rich or lean
mixture that the cat can't compensate for, and then monitors the
oxygen sensor voltage.
a. Rich to lean threshold voltage
b. Lean to rich threshold voltage
c. Low sensor voltage for switch time calculation
d. High sensor voltage for switch time calculation
e. Rich to lean sensor switch time
f. Lean to rich sensor switch time
g. Minimum sensor voltage for test cycle
h. Maximum sensor voltage for test cycle
i. Time between sensor transitions

On 11/17/2020 11:03 PM, Arlen Holder wrote:
> For the record, here's a description of what the computer checks for:
> <http://www.smogtechinstitute.com/smogtech/images/PDFs/OBDIIDataInterpretation.pdf>
>
> 1. The computer looks to see if the heated oxygen sensor voltage rises
> above 600mV and falls below 300mV and switches in less than 100ms.
>
> 2. When the cat is working properly there is almost no switching action
>
>
>

Oh Arlen the cat does not work It just sits on my lap all day. But you
is so smart anyway.

On Tue, 17 Nov 2020 23:35:03 -0500, wrote:

> Oh Arlen

Permanent plonk! (the fact such creepy disgusting people exist is scary)

Moving forward, I found this Mitsubishi emissions recall bulletin titled:
o Catalytic converter overheating - emissions recall campaign
o EMR-05-001REVII, December 2005, Model 2002-05 Lancer
<https://www.autocodes.com/uploads/mitsubishi/05-0001.pdf>

Purpose:
Some affected vehicles have incorrect parameters in the Powertrain Control
Module (PCM) software that may cause teh warm-up catalytic converter to
overheat and melt when the vehicle is repeatedly driven at wide-open
throttle.
o For all affected vehicles, reprogram the PCM as described in this
bulletin, regardless of the age or mileage of the vehicle.
o If an affected vehicle has a Service Engine Soon (SES) lamp on and
DTC P0421 set, also replace the warm-up catalytic converter, using
an exhaust manifold repair kit.

Affected vehicles:
2002-2005 Lancer models equipped with 2.0L engine.

Customer notification:
A letter will be sent to all owners of affected vehicles, telling them to
bring their vehicle to a Mitsubishi Motors dealer to have the PCM
reprogrammed at no change, and that if their vehicle has a SES lamp on for
failure of the warm-up catalytic converter (DTC P0421), they will also
receive a new warm-up catalytic converter.

In addition, owners will be advised that the new warranty coverage for the
warm-up catalytic converter will be extended from 8 years/80,000 miles to a
limit of 8 years/100,000 miles. Until February 28, 2006, replacement of the
warm-up catalytic converter will be free of charge, regardless of vehicle
mileage.

After February 28, 206, all owners (regardless of vehicle mileage) will be
entitled to the PCM reflash free of charge. However, warm-up catalytic
converter replacement will not be covered under the warranty if a 2004-2005
model has over 80,000 miles.
o P/N MN195802 Lancer 2.0L, 2004, Exhaust Manifold Repair Kit
(includes manifold with catalytic converter, gasket, and nuts)
--
Mitsubishi Customer Relations Department +1-888-648-7820
o I need to ask the owner for the VIN and give them a call.