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Discussion Starter #1
In reading thru these various HELP! I threw a P0420 code threads I see a lot of confusion and misinformation. And while folks like Cardoc have attempted to give you some practical help, I believe that unless you understand the “WHY”, you won’t appreciate the diagnostic or fix information he's offered up. So I’ve attempted to write this from a systems engineering perspective, and a look at how Subaru and others have responded to the mandates set forth by the EPA and other government agencies.

Cold Start:
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 O2 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 HO2S – 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.

Almost warm:
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.

Sensors & OBD-II:
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/MAP, 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.

Continuous vs Occasional Data Taking:
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 (HxCy) are combined with free oxygen and converted to carbon dioxide (CO2) and H2O. Oxides of nitrogen (NOx) are “reduced” to become O2 & N2 (& H2O 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.

The Once-A-Cycle Cat Test:
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 O2 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 typically 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 cycle failures required to call out a hard system failure.

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 Cardoc’s write-up, he addresses some of the factors that can short-life a good catalytic converter, and why so many people still have issues after changing them out. If there is interest, I can explore in more depth some of the chemical and engineering aspects of those events.

Cardoc’s link: http://www.subaruoutback.org/forums/66-problems-maintenance/49537-p0420-diag.html
 

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I read this with great interest. If I am failing smog and I DO NOT have a CEL or codes to that effect can I still have a bad catalytic converter? My mechanic said he thought I might and to expect a CEL in the near future but that was a year and a half ago. If folks WITH the code are reluctant to replace a CC, then you can see why I am left wondering. Could my CC be at like 66%, fail smog at idle, and never throw a code?

Oh and another question, what do you think the effect of burning engine coolant might have on a catalytic converter?
 

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Discussion Starter #3
Does CA still rely on a tail pipe sniffer rather than an OBD plug-in test? If you fail, do they give you the PPM value of hydrocarbons so that you can tell by how much you are over? Are they adjusting the test criteria for the year of mfgr, along with any derating for old age, or are they really holding your feet to the fire to meet a current new car standard? Can you ask for an OBD test rather than a sniffer?

According to the EPA, if OBD is clear, you pass. NY is a CARB compliant state, and they follow the EPA testing method. Is CA really different in this respect?

Head gasket leaks - I was going to go there if asked. Here's the short answer.

In the very very old days, cars used alcohol mixed with water to expand the temperature range. Today we use a derivative - double-alcohol to do this, as it's more stable in many ways. Ethylene glycol is an ethane molecule with two hydroxyl groups hanging off the ends. Carbon/hydrogen/oxygen. Simple enough, and although too much could be an issue, it wouldn't appear that a little bit entering into the combustion process should a big deal. The problem is the additives, and probably public enemy #1 are the various metal silicate compounds that some coolants contain. These bind to the reaction metals in the cat and the oxygen sensors and render them useless.

So, if you own a Subaru, I would think that a coolant that didn't contain silicates would be your best bet. However, what's in that Holts Leakstop that SOA wants us to use? I don't actually know, but silicates are often used in clotting compounds, so this might be another issue to consider.
 

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Discussion Starter #5
Cardoc, where are you? Is there a combination of OBD port PID's by which you could derive an actual hydrocarbon PPM level? I'll admit, it's not something I know about, but maybe it can be done?

I would have assumed that a hard number like 114 PPM could only be obtained using a gas analyzer.

It's been a while, Dianer, since I rode around with my laptop connected, but I seem to remember that when fully warmed up (coolant temp reading of around 200' F), that idle RPM was something like 600-625 or so? That makes your 700+ a bit on the high side, but not crazy high.

(OK, weird... I came back to correct my English, and your post of followup questions disappeared...)
 

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I am sorry Fibber, I removed the post I responded with because it sounded rant like and incomplete. But yes, it failed in '09 at 713 RPM with HC 114 PPM. Thank you for your post and response.

Ok I found the '11 record, it failed at 713 RPM with HC 156 PPM.

I think I know what you mean by a sniffer, they used them when I lived in Oregon in the '80s. I have not seen anyone here using a long wand inserted into the tailpipe. It appears they are working with a computer like those used for engine diagnostics.
 

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Discussion Starter #7
No worries. I sense your frustration. It's why I spent a good part of my afternoon writing this long-winded post.
 

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Well it may or may not be useful for people with a CC code or CEL to know that it is possible to have real emissions problems without ANY CC or CEL but there it is. I did not mean to muddy the waters you were trying to clarify.
 

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Discussion Starter #9
Someone else from CA will have to weigh in here for a better answer on this. Looking up CARB Smog Testing, I still see tailpipe gas analysis testing in addition to OBD-II port testing.
 

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As stated, different manufacturers use different programming. In programming, the math calculations vary. I have not looked at specific values in relation to HCs to find a specific level of output. The sensor measures oxygen and the lower the values reported from the O2 the higher the CO2 levels tend to be which indicates low HC. I posted a couple videos on my YouTube channel, cardocm, and show the efficiency of E85 running without CATs and then a comparison running 93 octane without CATs. In the video I show scan tool and 4 gas data to indicate what the ECM see verses actual output so no one could say I skewed results.

I look for the average of .700 mv or higher indicating a failing CAT or .500 mv indicating a working CAT. Depending on the ECM programming for CE, FE and then the new various ZEVs, the data from the O2 will vary.

I'll post video links and a link back to a new post in P0420 Diag. I had another yesterday that had CATs replaced but the originating problem was never looked for and they are failing again.
 

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Discussion Starter #12
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.

Contamination:
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 throw a front sensor heater or AF mixture code right at the 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, you don't have that extra minute. And that is often the problem with generic 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.

Stay tuned for the next chapter!!
 

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Discussion Starter #14
So much to cover, Doc! We have both pre-ignition (as in carbon hot spots) and detonation (poor air-fuel mixing/stratification or just lousy fuel). From there we can look at the knock sensor response and habitual engine retard, etc.

This could become a life's work!
 

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Going on 28 years of building and repair and my head is chock full of information.

Also, octane and alcohol effects, oxygenating agents, anti-gels, etc..
 

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Doc, if you are inclined to put your technical writing hat on, feel free to jump on in. All I ask is that you concentrate on the WHY. People are constantly bombarded with "Do This - Don't do That" type posts. I know that I don't respond well to being told to change my ways unless I clearly know what's in it for me. We can help people to think outside of the box and experiment for themselves if we give them a bit of knowledge to work with. That's my goal in writing this, and you are more than welcome to help.
 

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:2cents:

Combustion

Combustion is, in simple terms, burning of a fuel and oxygen mixture that creates heat and energy. A camp fire is simple combustion. You have a fuel, wood, and the surrounding air has the necessary O2 element. When combined and its ignited with a spark or excessive heat, it creates energy through the production of more heat. The exhaust of the fire is its smoke. It is the molecular change in elements due to the reaction that takes place between the carbon, hydrogen and oxygen contained within the fuel and surrounding air.

A solid in the beginning, combined with O2 and a heat source is converted into energy, CO2 and H2O. Other elements are present within the exhaust of the fire in various levels. The makeup of the carbon in the fuel and contaminates in the surrounding air will effect a change in the chemical reaction induced by the heat and thus create additional molecular change, sometimes low, at other times high, within the combustion process and effect an alteration of chemical and particulate matter in the exhaust. One of these elements is Nitrogen which when burned in combustion creates nitrogen oxides.

A large factor in the effectiveness of the combustion process and how much energy and particulate matter is derived from it is compression.

Compression

Compression is a reduction in size. It is also the method by which molecules are squeezed or pressed together in tight form under pressure. In computer sciences, you compress a file to reduce its usable space in memory. In an engine it is the process of applying pressure to squeeze molecules together in to a small space that they would under normal circumstances not exist.

Force anything into a tight space under pressure and it will push back. Its only natural. An easy example would be to use a garden hose and attach it to a container that will not expand and turn the water on. The water will fill the container and as it flows in it will push against any air within the container compressing both. When the water and air becomes compressed to its allowable threshold determined by the pressure applied from the hose, it begins to push back on the hose, stopping flow. With pressure on the container, remove the hose quickly. What happens? The water and air pushes out and spews from the container as both expands, reversing compression, releasing energy.

In an automobile engine, the fuel and O2, and other gases as they may be present in the air, are compressed into a small space. Since the mix is under compression, its natural that the mix will push back.

Add in combustion with compression of fuel and O2 and that creates expanding energy confined in a small space that pushes back harder because combustion releases energy through a mass of heat. It is an conversion of the energy from a chemical to thermal.

Temperature

Combustion creates heat and heat management is key in managing the amount of work obtained. With combustion engines, the fuel and air are compressed, then combusted creating expanding heat and energy. The amount of heat created is factored by the type and volume of fuel in direct ratio to the available O2. Low amounts of O2 means the combustion is quick and heat levels and the energy output is high. High amounts of O2 means combustion is slow with less energy created.

What is necessary for effective energy output in an engine is the proper mix of fuel and O2. This mix is the stoichiometric equation 14.7:1 or AFR. 14.7 parts O2 to every 1 part gasoline. The perfect mix where all available O2 is consumed and the fuel is burned is 13:1, but this would create too much heat under constant load and risks damage to the engine. So to maintain heat control and effective energy production simultaneously, more fuel is added to cool the combustion and avoid overheating the cylinder walls, pistions and valves. Adding air would also cool the combustion, but the slower burn would allow for more heat absorption in the surrounding metals.
Without getting in to a physics lesson that would require formulation and math, put in simple terms, the AFR is used to control combustion temperature to allow for energy production, safe temperatures and emission control. Adding fuel to the mix prevents pre-ignition and detonation while increasing the rate of combustion. Reducing fuel can also cool the combustion temperatures but decreases the rate of combustion allowing for quicker heat absorbtion in to the surrounding metals. So, the stoichiometry of gasoline is such that in everyday operations of an engine, energy is created and heat transfer is reduced.

This brings us to the Catalytic Converter function since the ratio allows for higher hydrocarbons and nitrogen oxides to escape the combustion chamber and exit to the exhaust pipes. Catalytic converters are designed to work best when the exhaust gases passing through them are the result of nearly perfect combustion, but automobiles are not programmed to operate at perfect combustion. Close, not perfect.

Fuel

Most gasoline blends consist of a combination of heptane, octane, alkanes, plus additives including detergents, and possibly oxygenators such as MTBE (methyl tert-butyl ether) or ethanol/methanol. These alter the stoichiometric ratio depending on the percentage of these additives in the gasoline, with most of the additives pushing the ratio down toward "lean", or having a high ratio of oxygen. Oxygenators add more oxygen to the combustion in liquid form that is released at time of combustions. MTBE fuel additives can cause a stoichiometric ratio to be as low as 14.1. And depending on the amount or percentage added, even lower.

Using an oxygen sensor in the exhaust stream to measure the available oxygen in the exiting gases helps to compensate for this change by transmitting the value to the ECM which will alter the fuel delivery accordingly to get the ratio back up to 14.7. The system adds more fuel by lengthening the time the fuel injector is open. The ECM reduces the amount of fuel as the ratio reflects higher than stoich using the same sensor's data. In this manner, combustion and temperature are controlled and the energy output is maintained as close to consistent as possible. Maintaining a constant fuel trim is impossible because of the many factors which effect the combustion temperature changes. Of these, intake air temperature, fuel temperature, compression ratio, volume of air (forced induction or NA), volume of fuel, temperature of the surrounding metals in the cylinder and backpressure from the exhaust are just a few. Each may seem a small value, but when combined these, and others, determine the overall performance of the engine and effectiveness of the combustion process.
 

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Discussion Starter #18
Cardoc brings up some good points in his review of the combustion process, and I’ll admit that I glossed over and greatly simplified some of it earlier because I didn’t want to overwhelm you all. But some of it is worth looking at a little deeper, because in a strange way it may address a question raised earlier by Dianer in his comment about OBD reporting a “pass”, while CARB calls his car a “fail”.

If you want to read thru one of his source files, go here:
Air?fuel ratio - Wikipedia, the free encyclopedia and as Cardoc said, be prepared for a lot of math!

Simply put, we are not dealing with straight octane (a perfectly uniform eight carbon string) and pure oxygen, but a fuel mix who’s composition changes per EPA regulation several times a year, and ‘air’ which is predominantly nitrogen. Nor are we dealing with an engine that is running at exactly the same RPM or under the same load at all times. It’s easy to tune a steady-state system for low fuel consumption and near zero emissions, but you actually drive your car.

Obtaining perfect combustion kinetics would likely destroy your engine on the first acceleration run, so we intentionally ‘detune’ it for survival. And then we expect the catalytic converter to deal with what we dump on it. In fact, the only time we really come close to a perfect stoichiometric mixture is during light load conditions at moderate road speed. Now remember back in post #1 when I described the conditions under which the Catalytic Efficiency test is run? Exactly! We run the test under ideal conditions for the best possible outcome, but know that most of the time the converter is under a lot of stress from non-ideal cycling.

Your engine has to operate from 500 RPM to over 6500 RPM. Physics tells us that the flame front propagation of burning fuel takes place at a certain speed. If it wasn’t for the fact that we can vary injector, spark, valve timing and other parameters, it simply wouldn’t work. The piston would be long gone in it’s intended travel before we burned the charge, or worse, we’d burn the charge when the piston was still rising. All engines have a sweet spot of operation, and where we run the Cat Eff test just happens to be one of them. Idle probably isn’t, as the engine sits on the threshold of stalling, making minimal horsepower and torque. So I’m not that surprised that an older engine might pass the OBD test, yet show a little over the State’s mandate at idle. I think I’d go into the test after running a few tanks of premium fuel and a bottle or two of Techron or Seafoam cleaner to better my odds of a positive outcome.
 
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