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An Engineering Analysis of the P0420 / P0430 Catalytic Efficiency code

42356 Views 12 Replies 7 Participants Last post by  Jlancehi
Multiple times in the past I've tried to jump in and offer some insight into how to handle P0420 / P0430 Catalytic Efficiency codes, both on this board and on a few others. There is a lot of confusion and misinformation out there. I thought I'd consolidate my understanding in one place, and maybe others with specific insight will share as well. Forgive me in advance if I don't have it all letter perfect, but it's still got to be way better than what we currently have. And while many folks may attempt to give you some practical help, I believe that unless you understand the “WHY”, you won’t appreciate the diagnostic or potential fix information you are being offered. I’ve attempted to write this from a systems engineering perspective, and a look at how Toyota and others have responded to the mandates set forth by the EPA and other government agencies.

In the beginning:
What happens when you start your car from cold? For the first 300 seconds or so, the system is in “open loop” operation. That means very little is being monitored, and the engine air-fuel ratio is determined from a ‘look-up’ table of factory stored and some learned-modified values. It’s better than running with a carburetor, but along those lines when it comes to clean exhaust. During this time, small heaters are running within the front wideband Air-Fuel Ratio (fancy oxygen sensor) sensor and the rear O[SUB]2[/SUB] sensor, because much below 550’F, these gizmo’s don’t work all that well. That’s the ‘H’ you often see in the more technical description of these components (as in HO[SUB]2[/SUB]S – heated oxygen sensor). The cat itself is being heated by the hot exhaust gases, but that heat wouldn’t transmit to these externally mounted sensors fast enough for them to work in time for the switchover to “closed loop” operation.

Interestingly enough, the EPA’s draft OBD-II docs actually called for electric blankets around the cats to speed their heating to enable a 100 or 200 second changeover period, but that would have required a monster alternator. I’m not really sure if there are any heated cats in production today. The EPA did hold out some nice labels (like ‘super low emission vehicle’) to those that had faster warm-up cycles. Maybe if we ever move to 42V electrical systems this would be practical. There are some P0xxx codes for heated catalyst failure, but I doubt you’ll ever see them called out.

[FONT=&quot]Almost warm:[/FONT]
At the 5 minute mark, the cooling system should be reporting at least 145’ F, the heaters on the sensors should have done their job, etc., and if all sensors report seemingly valid data, the system switches over to “closed loop” operation. Now there is a steady flow of feedback data on throttle position, RPM, atm pressure & air temp for calculating mass and thus actual oxygen content of the intake air, front AF sensor’s look at exhaust purity, knock sensor, etc., and from this we calculate loading and injector duration needed to keep the exhaust cycling between slightly rich and slightly lean. We need that cycling in order for the 3-way catalytic converter to do a proper job of dealing with all three of the chief pollutants the EPA wants to control: Carbon monoxide, complex unburned hydrocarbons, and various oxides of nitrogen.

Why cycle? Surely with today’s management systems we could keep it dead nuts at an even 14.7. The problem is that oxidation and reduction are somewhat inverse reactions, and we need periods of lean to gather up and store that ‘excess’ oxygen that we will need when we go rich. But we also know that if we stray too far from stoichiometric ratio, bad things can happen (more on that later). So could we do better than live with this intentional cycling? The short answer is yes, but with more complex hardware that we may see with OBD-III and the true ‘clean air’ engine.

If you were to look at some of the live output data from the OBD monitor port, you’ll see things like Long Term Fuel Trim and Short Term Fuel Trim. This is part of the immediate self adjusting and longer range table building – the Learning Mode, that the system does to improve the precise control around the magic 14.7 air-fuel ratio. And this is exactly the data that you kill off when you attempt to clear a code by disconnecting the battery! You torture your converter every time you do it this way. And on newer systems that don’t use a separate IAC channel and instead rely on a learned value for the main throttle plate, you may have days of severe idle instability and misfires as a result. Buy a reader and clear codes without wiping out these essential stored values.

[FONT=&quot]Sensors & OBD-II:[/FONT]
Let’s talk about sensors themselves for a minute. How reliable is the prognostication in a thrown code? In general, it’s pretty good but far from perfect. A big part of this is the balance between cost and complexity. In an aircraft, you might have 3 sensors in each spot, and the computer uses balloting to determine if there is a real fault, or just bad data. In your car? Other than the drive-by-wire throttle (which contains 2), everything is single. So if an AF sensor is out of spec, it will throw off the mixture. The system really has no way of knowing if it’s a sensor problem or an actual injector, computer, temperature, MAF, etc., problem. By polling all other sensors and using a lot of deductive reasoning to determine that SOMETHING in all the reports just makes no sense, it throws a code. It may or may not be the right response, but something is probably wrong! OBD operational software is up to the manufacturer to write, and some simply have better programmers than others.

And this is where examining the data stream and a keen, experienced eye come into play. Like it or not, in the end it will come down to a certain amount of experience in reading the tea leaves, looking for other systems on the car that might be causing the failure, performing some parts changing, and re-evaluation. Again, I believe that OBD-III and manufacturer proprietary on-board testing and some redundant sensors will help address this.

[FONT=&quot]Continuous vs Occasional Data Taking:[/FONT]
Much of the engine’s sensors are under continuous monitoring, and so codes for most subsystems could be thrown at any time. Two key subsystems are not – Evaporative Emissions and the infamous Catalytic Converter Efficiency. These are evaluated only once per drive cycle, so the opportunity to pass or fail for these are limited to a few precious minutes. For Evap, this takes place when the system is stone cold, about 5-6 hours after shutdown. The Cat Efficiency test takes place sometime after going over to closed loop operation, but only when certain steady state conditions are met. There is a designated speed range, throttle position, etc., that must be met first, before the testing period begins.

3-way catalytic converter performs two primary reactions, known as oxidation & reduction. In the oxidation reaction, carbon monoxide (CO) and unburned hydrocarbons (H[SUB]x[/SUB]C[SUB]y[/SUB]) are combined with free oxygen and converted to carbon dioxide (CO[SUB]2[/SUB]) and H[SUB]2[/SUB]O. Oxides of nitrogen (NO[SUB]x[/SUB]) are “reduced” to become O[SUB]2 [/SUB]& N[SUB]2 [/SUB](& H[SUB]2[/SUB]O if we also have hydrogen remaining from the oxidation reaction). In a monitored reaction chamber, we would ideally have sensors that report on the efficient conversion and percentages of each constituent. Nice, but that would add at least $5k to the cost of your car! So instead we simply monitor the comings and goings of free oxygen in the exhaust, do some fancy calculations to determine the “Catalytic Efficiency”, and call it a day.

[FONT=&quot]The Once A Cycle Cat Test:[/FONT]
In the Catalytic Efficiency test, the ECU actually momentarily leans out the mixture by up to 30% and evaluates the waveform output response of the rear (post cat) sensor in relationship to that of the front AF sensor. It then returns to the stoichiometric point and watches the rear settle out. It then richens the mixture by up to 30% and compares the waveforms again. So it is monitoring the cat’s ability to store up excess oxygen during a lean period and hold on to it, and then release it on demand to cope with an extended rich mixture condition. How much oxygen should it be able to hold and release? Oxygen storage is a separate oxidation/reduction reaction that occurs using a honeycomb section of the cat coated with Cerium or other similar fast reacting metals. Are the actual precious metal surfaces (Platinum & Palladium mainly for oxidation, Rhodium for reduction) where the exhaust constituents meet up with the freed O[SUB]2[/SUB] properly reactive enough that all this oxygen will be consumed, and the exhaust smell like roses?

Each manufacturer matches a cat capacity with a given engine, and determines a maximum amount of oxygen storage they expect to have available for rich mixture oxidation when components are new and all surfaces are fresh. The failure criteria is typically set at 65% of this max value. Again, it’s not an absolute reading, but a calculation based on the comparison of two waveform. The first time it fails this test, a pending code is logged. If it passes the next time (full cold start cycle), the pending code is erased. Generally, it takes two successive failures to log a hard fail and set the light. Interestingly, this criteria doesn’t seem to be absolutely hard and fast, and reading thru test methodology in Ford, Honda and other manuals I see variations in % and number of successive drive cycles.

So, a number of thoughts come to mind:

1) As it is a comparison, the health of the AF sensor could be just as important as the cat and the rear sensor.

2) Rear sensor issues could provide skewed results.

3) Those that have reported intermittent fails when they clear the codes probably don’t have dead cats – just marginal. Fix minor things, and you might be on the road to recovery. Looking at waveform data would help you assess how bad the cats are.

4) Could you screw the test? Sure. Have a heavy foot in the middle of the testing sequence, and you could make it look a lot worse than it really is. How will you know when the test is running? Tricky…. I’ve sometimes felt a sag on a straight, steady state drive a few moments after the transition to closed loop, and suspected that the ECU was in test mode. This obviously wouldn’t be a smart time to floor the accelerator to pass someone! I would think that if you were running a live monitor, you would probably see a sudden change in fuel trim corresponding to the initiation of the test sequence.

5) Some people mention that their car seems to run fine despite failing this test. Probably true, as this test is simply intended to report the state of affairs. The presence or absence of a working catalytic converter (assuming it isn’t clogged, etc.) will have no impact on the running of the engine. However, if the cat test failure is directly attributable to other factors, such as a bad front sensor, a bad thermostat, or a dozen other issues, then the cat test failure just becomes a symptom of a larger problem that will eventually impact total vehicle performance.

In future sections, I'll addresses some of the factors that can short-life a good catalytic converter, and why so many people still have issues after simply changing them out.
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The future came fast! Actually, I wrote this today for another site...

So what exactly is a Catalytic Converter, and what kills them?

A catalyst is something that promotes a reaction, but in most cases doesn’t enter into the chemical reaction itself. The promoting material is either unchanged or continuously renewable. It’s all about a field called “Surface Science”. For instance, unburned hydrocarbons will adsorb to a clean platinum surface when the conditions are right. Different from absorb, this is about ‘sticking’ to the outer surface at the atomic level. On the metal surface, the HC’s combine with liberated oxygen and are burned, giving off a lot of heat and benign byproducts.

All this relies on the right conditions.

First, the catalyst surface must be clean and uncontaminated. If your engine has been running particularly rich, HC buildup can occur. Without the proper surface dynamics, sufficient oxygen will not be stored on the cerium, or the liberated oxygen could simply pass on by the HC’s on the platinum. Either way, the system no longer works with the same efficiency. Silicon, lead, phosphorus, zinc and other elements from various chemicals can permanently seal (passivate) the metal surfaces. A common source of silicon is various gasket sealers, and anti-freeze (coolant) that contains metal silicates. Having a head gasket issue? Hopefully you have a low or silicate-free coolant in there.

If you get excessive HC build-up, you might be able to burn it off with an ‘Italian Tune-up’, a tank or two of premium with a strong detergent package, an oxygen-rich fuel (a tank of E85 maybe - but only recommended if your car was built to run this...), or a good fuel additive. This is particularly true if you develop this problem after repeated short drives in which you barely get up to temperature. More on that a few paragraphs down. With other chemical contamination, you may not be able to recover functionality.

The Right Cat:
We need the right amount of internal surface area and the right balance of cerium, platinum, palladium & rhodium to match the characteristics and volume output of the engine. Yes, cats are carefully designed and tailored to the engine and application. This includes not only what is inside, but also the surrounding airflow and heat retention characteristics. Even the physical shape can matter. It’s a finely tuned coexistence, and among the reasons why “universal” converters often don’t work particularly well.

Is Everything Else Working:
The front AF sensor and ECU must properly cycle the lean-rich balance to fuel a precise oxidation-reduction dance to eliminate all three primary pollutants. The ECU relies on the input of a dozen sensors to determine the injector dwell and spark timing. It can handle some system variation, but then you subject it to a mode of continuous compensation. For instance, normal coolant temperature may be around 200’ F. If you have a bad thermostat and the engine is forever at 160’ F, it will run below peak combustion efficiency. Both the temperature sender and the front AF sensors will send this information to the ECU, which will try to compensate. This may result in skewing the STFT & LTFT way off the mark, eventually sending sufficient HC’s to the cat to dull either the conversion material, or the front & rear sensors. Now you start throwing seemingly unrelated codes for AF mixture or Cat Eff, when the real cause was a stuck thermostat.

Internal Temperature:
The temperature of the converter must be managed to keep it around 600’ F. Too low and conversion efficiency drops. Too high, and metal coating delamination from the ceramic and/or meltdown can occur.

Notice that many cats are located adjacent to the exhaust manifold rather than underneath on newer designs? It’s all about light-off speed – the point where efficient reactions begin to take place. What about heat shields? They serve two purposes – to keep things nearby from getting burned, and for keeping sufficient heat in. Guess what is more likely to happen when you opt to remove those pesky shields from the pipes or the cat?

While some operating temperature information can be derived from the sensors, it’s mainly used for determining if the preheater function is operational. Remember that requirement for operation at 300 seconds? If the sensor isn’t at temperature because the heater resistance is too high, the sensor reading will be way off of what the ECU is expecting to see. It’s pretty common to thru a front sensor heater or AF mixture code right at the moment of open-to-closed loop switchover if the sensor isn’t fully operational yet. Maybe if it was queried another minute later, all would be well. Unfortunately, the EPA didn’t give you that extra minute. And that is often the problem with generic or universal sensors. Ever wonder why there are so many sensor part numbers? Yep, just like cats, they are very specific to the application.

So in an ideal world, there would be an internal cat temperature sensor, and a way of regulating cat temperature to keep it in the right range. Unfortunately, it runs open loop. We don't regulate the engine to keep the cat happy, we run the engine and hope that the cat stays happy. If the engine is perpetually cold and sooty, the cat may stay cold and is likely to clog. If you dump excessive unburned fuel into it, it will overheat. And that brings us to MISFIRES and flashing CELs.

I’ll write more when I get a chance.
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So, for a properly running Sienna, what should the oxygen sensor readout (voltage?) via OBD2 scanner look like? How about for a Sienna with cat or sensor issues?
Thank you very much for this very informative post. My 2005 Sienna is in the shop right now because I'm having its catalytic converter replaced. My car has 130,000+ miles on it and I'm having both the converter and the sensors replaced. Thanks again!
Thank you very much for this very informative post. My 2005 Sienna is in the shop right now because I'm having its catalytic converter replaced. My car has 130,000+ miles on it and I'm having both the converter and the sensors replaced. Thanks again!
Having the CAT and the sensors - WHY ? Either the Cat is bad or it isn't , the sensors are bad or they aren't. Sounds like a shotgun approach. Then again maybe you figured they were in there and just let em do it all.

Changing the Airfilter, the Oil, the PCV valve, and the spark plugs if you have not done so can resolve the problem.Cleaning the Mass Airflow Sensor or replacing it can also solve the problem according to some sites.
The condition is based on what is going in and what is coming out oxygen wise. If the upstream sensor is being fooled by bad stuff going in the output sensor will compare the reading to an upstream that is blinded by all the bad garbage.
This is excellent information. Based on this this and some other posts I applied a $9 bottle of Techron. This cleared my pending code and allowed my 2002 Sienna to pass Texas emission testing. I can't thank you enough. I may in the future need to replace the Cat, but in the short term it is a huge relief.
You're welcome! I wrote this because I was in the midst of dealing with an old Subaru that occasionally thru a P0420. I kept the Grim Reaper at bay for close to three years by tossing in a container of Techron, CRC's "Guaranteed to Pass", Seafoam, etc. periodically, or when it failed. I'd add a bottle and drive it hard for a day or two to burn off the crud, then clear the code. In the beginning I could get 4-5 months relief, and head for an inspection every 6-8 months or so just to be sure I always had a year of 'insurance'. By last winter I was shopping for a Cat, and finally decided to sell the car.
I would think the CAT test, if in process, would be postponed if you were to accelerate. I can't believe the test would be allowed to cause noticeable performance issues. I've never experienced any hesitation.

Apparently, the CAT is what's called a "Non-Continuously Monitored Sub-System" according to RepairPAL ...

"Non-Continuously Monitored Sub-Systems"

There are five or more non-continuously monitored sub-systems. While the vehicle is being operated, and even while it is parked, the PCM is testing the performance of these sub-systems during very specific operating conditions. These conditions may only occur once every 24 hours, such as "cold starting" after a vehicle has been parked for at least eight or more hours. These sub-systems are:

Oxygen Sensor Heater Monitor
Oxygen Sensor Monitor
Catalytic Converter Monitor
EGR Monitor
EVAP Monitor
Secondary Air System Monitor
Air Conditioning Monitor
Heated Catalyst Monitor

When you have a P0420 or P0430 with possibly with other codes ... obviously, as fibber2 already stated, the other codes should be eliminated first, and the engine should be in good tune, before attempting to diagnose the CAT codes.

That was the case with both my 2004 Toyota Sienna LE, (and my 2003 Pontiac Vibe GT - really a Toyota Matrix XRS). I had several codes, which I fixed ... and was left with the P0420 and P0430.

I used the diode trick rather than CAT replacement on both cars ... and after about 2000 miles on the Sienna, and about 7500 miles on the Vibe, still no codes!
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Yes, the Cat Efficiency is one of the non-continuously monitored tests. And I'm sure you've walked into your garage at night and heard the Evap pump running and venting. That takes place 5-8 hours after shutdown, when the system is stone cold.

I doubt that the system has the ability to toss out a result and rerun the test if you were to have messed it up by accelerating hard. And you'd have to do this twice in succession to fail, so it's unlikely to happen that way. In your favor is that the test is short, and if you've got reasonable reserve capacity in your cat, you shouldn't have problem with a sudden jab of the accelerator anyhow.

Remember that we are dealing with a 1996 standard here, implemented with the available hardware (microprocessors and coding) of the day. Even if a manufacture had the smarts with today's computing power to implement a continuous monitor or a 'toss this result and try again' approach, they'd be accused by the EPA of cheating. Not going to happen. Honda did that once around 2000 and got caught. Paid a pretty hefty fine, IIRC.

In 2008 I sat in the audience at a panel discussion on automotive applications at a major conference on electronics reliability discussing "Draft OBD-III". Here we are 8 years later and still building to OBD-II.

Beat the system? Sure, there are lots of ways. But as a resident of Planet Earth and interested in the air I breath, I certainly wouldn't want to return to the stench of the late 1960's. If you've ever traveled to China, you know what life could be like if we all tossed our emission controls. Still, I understand the economic impact of cars that were released by their manufacturers with marginal systems that we consumers are now responsible for fixing, and they turn their back on us.

My cleaners helped make the system run legitimately well for a while. Some have taken the cat off and cleaned it internally with good results.

I knew of one guy who built an electronic oscillator that presented the ECU with a totally contrived waveform. The diode and resistor trick attenuate or delay the signal enough to trick some ECU's. And there's ways of pulling the sensor head out of the mainstream flow, to keep it from seeing peak lean or rich situations. But many of the newer CAN systems look for more than just a wave. It must be sync'ed to and properly respond to the wide band sensor and other factors, because it plays into the EPA fuel economy testing, and so it will detect the fudge and fail them. Most of these approaches stand a chance of working if the Cat still has some functionality. It might be at 50% and not at the EPA required greater than 65%. But many of these hacks will fail if the cat is totally dead.

The only fool proof fix would be the guy who could manipulate the code and reflash his ECU to simply ignore a fail! There are good tuners out there who can re-calibrate the wideband sensor to accommodate short cold air intakes, turbo & superchargers, large flow injectors, etc. I'm sure 'fixing' the downstream sensor has been done....
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It just seems to me that there's got to be a better (cheaper) way to eliminate emissions than using $500 - $1500 Catalytic Converters.

I've worked in inner-city ministry for almost 30 years ... so I come across a lot of people who may have an older car, that maybe they've purchased for $500 ... but even though they make work, they simply don't have that kind of money to keep it running properly. Hence, there are some things they just ignore (like replacing CAT's).

You just wonder how many vehicles in America (let alone the rest of the world) really comply with current emissions standards.
I hear you! There is no such thing as a cheap (or light) car today. The quest for safety, emissions control & fuel economy had created a monster.

Part of what I don't understand is exactly why an OEM cat costs over $1k. Yes, there's some precious metal in there, but not THAT much. Aftermarket cats are far cheaper, and reportedly reasonably good these days.
So, for a properly running Sienna, what should the oxygen sensor readout (voltage?) via OBD2 scanner look like? How about for a Sienna with cat or sensor issues?
Anyone have an answer to this? Thanks!
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