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

[Arlen Holder]

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

Failed smog o 180,000 miles o 1 DTC with all registers set o P0421 below threshold, bank 1 o Lambda sensor OK o CAT OK o EVAP, MIS, FUEL, OK o Live data page 1 o Live data page 2 o Live data page 3 o Live data page 4 o What values do I look for?

QUESTION: o What do I look for in terms of "live data" to help diagnose?

[hubops]

. . . . Oh ... Arlen ..

.. can you post your favourite BlueGrass band/song ?

Carry on ..

John T.

[Scott Dorsey]

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

[Arlene.Holder]

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.

[gfretwell]

Lancer?

[micky]

Does your scanner give real time data?

Most scanners, all cheaper ones, only give the scan code, which you have.

Look it up online, see what it means, and see what remedies are suggested for it.

That will probably go faster than using the scanner even if it does give real-time data.

[micky]

Diagnostic trouble code (DTC) P0421 stands for ?Warm Up Catalyst Efficiency Below Threshold (Bank 1).? It means that the oxygen sensor downstream of your vehicle's catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should.

IMPORTAnt: Although the trouble code P0421 is related to your vehicle?s catalytic converter, it does not always mean that the converter is faulty. It could also indicate other issues, such as a circuit problem or a bad oxygen sensor.

And the cat is expensivie to replace, especially for nothing.

Here are some of the common triggers of the error code P0421:

Faulty Bank 1 catalytic converter Faulty oxygen sensor Issues with the oxygen sensor circuit, such as damaged wires or poor connections Exhaust leak An issue with the PCM, such as software in need of an update

[Arlen Holder]

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?

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).

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.

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).

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

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

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

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

And another stead 60 mph reading: o O2B1S1(V) 0.740 o O2B1S2(V) 0.640

I'm not sure the speed for this one: o O2B1S1(V) 0.460 o O2B1S2(V) 0.120

Here's another reading, at a steady 20 mph: o O2B1S1(V) 0.700 o O2B1S2(V) 0.400

And another at an unrecorded speed: o O2B1S1(V) 0.820 o O2B1S2(V) 0.600

And another at an unrecorded speed: o O2B1S1(V) 0.500 o O2B1S2(V) 0.800

Here's one taken at 50 mph: o O2B1S1(V) 0.140 o O2B1S2(V) 0.760

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

Taken shortly thereafter at 55 mph: o O2B1S1(V) 0.820 o O2B1S2(V) 0.700

Another on the highway at an unrecorded speed (2500 RPM though): o O2B1S1(V) 0.140 o O2B1S2(V) 0.220

Another on the highway at 60 mph: o O2B1S1(V) 0.400 o O2B1S2(V) 0.160

Another on the highway at 60 mph moments later: o O2B1S1(V) 0.620 o O2B1S2(V) 0.300

Another on the highway at 60 mph: o O2B1S1(V) 0.120 o O2B1S2(V) 0.180

Another on the highway at 60 mph: o O2B1S1(V) 0.440 o O2B1S2(V) 0.240

This one at 1,000 rpm (probably idling): o O2B1S1(V) 0.220 o O2B1S2(V) 0.840

Still at 1000 rpm shortly thereafter: o O2B1S1(V) 0.060 o O2B1S2(V) 0.760

Another at 1000 rpm: o O2B1S1(V) 0.600 o O2B1S2(V) 0.480

Yet another at 1000 rpm: o O2B1S1(V) 0.240 o O2B1S2(V) 0.140

Here's one at 10mph and 2300 rpm: o O2B1S1(V) 0.820 o O2B1S2(V) 0.780

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.

Only one bank, thank God. :)

BTW, I just checked. It's a 2004 Mitsubishi Lancer ES, 2.0liter SOHC 4G94 engine

[danny burstein]

[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

[rbowman]

?v=fdTiNAaCE74 Bluegrass is overrated.

[Dean Hoffman]

[Arlen Holder]

For the record, here's a description of what the computer checks for:

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.

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

2. 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

[Arlen Holder]

According to this discussion on the oxygen sensor reading:

1. At the 14:1 stoichiometric ratio we should get a reading of 450mV, but the pre-cat oxygen sensor will fluctuate around 500mV in use.

1. The post-cat oxygen sensor should be more steady at around 500mV since it's reading "cleaned up" exhaust.

1. However, if the pre-cat sensor is receiving too rich a fuel mixture, it will read a fairly steady 1.0V while too lean will read near 0V.

[Arlen Holder]

This explains the pressure waves:

1. The oxygen sensor monitors the content of the pressure waves as they pass the sensor. With each pressure wave the sensor sends a rich or lean mixture voltage to the engine controller.

1. At temperature, the sensor changes resistance as the air/fuel ratio changes. But instead of a gradual change, it switches very quickly from low resistance (less than 10 K ohms when the mixture is rich), to more than 20 K (ohms when the mixture is lean).

2. The engine controller supplies a base reference voltage of one volt to the sensor.

1. Lambda1 is the reference voltage of 450 mV, which represents an air-fuel ratio of 14.7:1 or stoichiometric.

2. The sensor is capable of generating a one-volt signal when there is no oxygen in the exhaust stream.

[Arlene.Holder]

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

[Arlen Holder]

This is a pretty good video (IMHO), on how the rear sensor works: o

No wonder my sensor readings jump around; it's a narrow band sensor! o Rich will read around 900mV; lean will read around 100mV

And it's tested for seven things a. Rich to lean switch time b. Lean to rich switch time c. Frequency of sensor transitions d. Rich to lean sensor threshold voltage e. Lean to rich sensor threshold voltage f. Low sensor voltage for switch time calculation g. High sensor voltage for switch time calculation

Under acceleration, the mixture will be rich (reading around 900mV). o When we slow down, the mixture will be lean (reading around 100mV).

While idling or coasting, the ECU will constantly change the air:fuel ratio between slightly rich & slightly lean to analyze how the rear sensor responds.

The rear sensor shouldn't switch frequently even if the front sensor does, which indicates healthy high oxygen capacity of the catalytic converter.

Low switching speed is greater than 24 seconds, whereas high switching speed is less than 1 second (which would be indicating that the catalytic converter has a low oxygen storage capacity).

The oxygen sensor needs to be at around 315 degree Celsius to operate correctly, so during a cold start the rear sensor will be at around 300mV flat, but when it heats up (after about 15 to 20 seconds), it will start to switch between lean and rich.

While driving at a constant speed (constant load), the rear oxygen sensor should switch slowly (which is a sign of a healthy catalytic converter with high oxygen capacity).

When coasting, the rear sensor should drop down to a steady 100mV; when accelerating it should go rich at around 900mV.

If you stab the gas pedal four times, you should see four spikes from lean (100mV) to rich (900mV) of the rear oxygen sensor voltage.

[Paul in Houston TX]

If you have access to a laser temp device or can borrow one... they seem to be everywhere nowadays... if the cat is doing its catty thing then the outlet temp will be a lot higher then the inlet temp. Run it for a

15 sec or more at ~2000 rpm then measure the temps, ~300-400F inlet and 600-800F outlet. If the ratio is lower then it may not be catalyzing very well and will give high HC readings at the tail pipe. In the old days a good working cat would start grass fires. Now they are better shielded so try to get the temp reading of the cat and not the shield.

If it were my car I would also take the O2 sensors off and bench test them.

[Arlene.Holder]

Oh Arlen, you get me so hot when you talk about stabbing four times but you cum in two.

[Clare Snyder]

I know it's Arlen again - but I'll make ONE post.

The#1 O2 sensor will "clock" - between nominally zero and one volt. If there is anything wrong with the sensor it will throw another code. If it is too rich or lean it won't clock and will throw another code. If the #2 sensor is out of range it will throw another code. If the cat is good the #2 sensor will not track the #1. If the cat is bad the #2 will track the #1 and throw a P0421. A scanner (not necessarily a simple code reader) will allow you to "read" the 2 sensors to confirm - but the chances are better than 95% you have a bad cat. Does that piece of crap burn oil??? A LOT of those POS cars do due to either bad valve guides or coked up rings - and oil kills cats. Antifreeze kills cats too. So does a missfire (or anything that causes a "flashing" CEL) If you have a non-cantact thermometer check the pipe temp ahead of and behind the cat. A functioning cat should be hotter on the out side than the in side.

[rbowman]

It's long but it's interesting. Popcorn Sutton was the real deal.

Hank III's tribute.