Catalytic Converter Cleaning, and Other Scams...


May 3, 2016
If you ever experience trouble codes P0420 and/or P0430, both of which refer to catalytic converter efficiency issues on your vehicle, we recommend you keep two things in mind. The first is that many mechanics and repair shops can, and do, fix these kinds of issues honestly and professionally, and the second is that trouble codes P0420 and/or P0430 are gifts that keep on giving to the many dishonest mechanics and outright scam artists that continue to give the car repair industry bad name.

So, what do we mean by “gifts that keep on giving”? Quite simply, it means that dishonest mechanics and scam artists will do their level best to persuade you that they can clean out or regenerate clogged or damaged catalytic converters in ways that will restore their efficiency to the level of new units. This is saying a lot, but before we get to the specifics of how these scams work, we need to understand what catalytic converters do and how they work, so let us start by answering this question-

Why do we need catalytic converters?​

The short answer is that catalytic converters offer an effective method of removing harmful waste products that result from the combustion of automotive fuels, such as various compounds of nitrogen and oxygen, commonly known as NOx, from automotive exhaust gas. As a practical matter, "NOx" is a generic term that describes a wide range of oxygen/nitrogen compounds, with the exact composition of each specific compound depending on, among other things-

  • the type and quality of fuel used
  • the quality of the oil in the engine
  • the engine speed and load at the time a specific volume of NOX was created, and
  • the mechanical condition of the engine
Limited space precludes a comprehensive discussion of the role each of the listed conditions plays in the formation of NOx, but suffice it to say that-

  • all modern internal combustion engines can potentially generate NOX under some operating conditions
  • all modern engines are fitted with one or more systems that reduce the formation of NOx but at the same time, no mitigating system can completely eliminate the formation of NOx under all operating conditions.
Based on the above, catalytic converters have been legally required components of the exhaust systems of all road-going vehicles since 1975 and on other categories of vehicles soon after, which begs the following question-

How do catalytic converters work, exactly?​

The finer details of how catalytic converters work are exceedingly technical and complicated, and as such, we cannot delve into the complex chemical reactions that convert harmful and toxic substances in exhaust gas into water vapor and carbon dioxide here.

Nonetheless, in simple terms, all catalytic converters contain a porous core that allows the exhaust stream to pass through it relatively freely. In practice, the porosity is achieved by small-diameter channels that run through the length of the core, and the conversion process takes place on the walls of the channels that are coated with a variety of precious metals. These typically include platinum, rhodium, palladium (and sometimes trace amounts of gold in some specialist applications) in various proportions depending on, among other things, the engine displacement and the type and quality of fuel used.

In a fully functional exhaust after-treatment system, the conversion of exhaust components starts when the core of the converter, and by extension, the precious metals that function as catalysts, reach a temperature of around 500 deg F to 800 deg F, and up to around 1200 deg F when the engine is operating under heavy loads. As the exhaust gas flows through the catalyst-coated channels in the core, the chemical reactions that underpin the conversion process assist in maintaining the core's temperature, so in a sense, the conversion process is in part self-sustaining.

Two things become important at this point; the temperature of the converter’s core determines the completeness of the conversion of exhaust components, and the second is that the OBD II system in the vehicle cannot monitor the conversion process directly. The only way for the OBD II system to infer the efficiency of the conversion process is to monitor the output signals of the oxygen sensors that are located before and after the catalytic converter. Here is the short version of how this works-

Since the upstream oxygen sensor monitors the composition of the exhaust stream before it enters the catalytic converter, its output signals vary in direct response to changes in the oxygen content of the exhaust stream. So, if we assume that the catalytic converter operates at an efficiency level of at least 95% or higher, the output signal of the oxygen sensor at the converter's outlet should remain relatively constant at or close to the midpoint of the sensor's range, which is typically between about 0.1V and about 0.9V under all engine operating conditions.

As a practical matter, the engine control module monitors the output signal of both oxygen sensors, and based on changes in the output signal of the downstream oxygen sensor relative to the rapid fluctuations in the signal voltage of the upstream oxygen sensor, the engine control module will infer an efficiency value for the catalytic converter. In most cases, modern catalytic converters have a minimum allowable efficiency threshold of about 96%, and when the inferred efficiency value falls below this threshold, the engine control module will conclude that the catalytic converter is operating at or below an acceptable level of efficiency.

If this happens, one or more control modules will set either (or both) trouble code(s) P0420 and P0430 and illuminate a warning light on the dashboard to alert a driver to the fact that there are issues with one or more of the catalytic converters on the vehicle. Although these trouble codes typically do not produce noticeable drivability concerns, the sad fact is that the presence of these codes often offers dishonest mechanics a way to exploit an unwary car owner, so let us look at-

The nature of the cat cleaning scam​

Even though modern catalytic converters are extremely efficient, they do not last forever- even in cases where no mechanical damage to the converter occurs. In practice, an OEM or OEM-equivalent catalytic converter should last for around eight years or so but despite their long service lives, all catalytic converters suffer a progressive loss of efficiency over time as a result of the deposition of carbonized oil and fuel residues onto the substrate and catalysts.

This is a natural consequence of normal engine operation, but if a dishonest mechanic wants to scam a car owner, he will not tell the car owner that. Instead, he will tell the car owner that (by using any number of snake oil formulations) he can restore the efficiency level(s) of the underperforming catalytic converter(s) to the level of new units. Worse, though, some dishonest mechanics will even claim that they can restore the efficiency of severely or even completely clogged catalytic converters, which is simply not possible by any means short of replacing the defective catalytic converter.

However, like all good scams, the “cat cleaning” scam does have grain, albeit a very small grain, of truth to it because there are a few aftermarket products available that might actually remove at least some of the ashy compounds that form in a catalytic converters' core channels during normal use. However, there is a major caveat to this statement, and it is the fact that-

Efficient catalytic converters don’t need cleaning​

The above statement may seem to be a self-evident truth, but on the other hand, the above statement covers a lot of ground, so let us break it down into more manageable pieces, starting with explaining the concept of-

Catalytic converter Efficiency

Modern three-way catalytic converters actually contain two distinctly separate cores whose channels are coated with different amounts and combinations of catalysts (precious metals), with each of the two cores converting different harmful substances into less harmful or innocuous substances.

Collectively, though, the two cores remove more than 99% of the harmful components of exhaust gas during the first 4 000 miles (or so) of use, after which a gradual but progressive degradation of the cores sets in. In practice, however, in the absence of other issues such as, excessive oil consumption, or persistent rich-running conditions, it typically takes several years before a catalytic converter reaches its minimum allowable efficiency threshold, which can be as high as 96% on most modern vehicles.

Thus, for as long as one or more control modules do not set catalytic converter efficiency trouble codes, it is safe to assume that the catalytic converter(s) are operating above their minimum allowable efficiency thresholds, and they are, therefore, by definition, deemed to be efficient by the vehicle’s OBD II system, which brings us to-

Why cat cleaning is a scam​

Most of the substances that can “miraculously” restore the functionality of catalytic converters are nothing more than completely ineffective mixtures of various types of snake oil. On the other hand, the few marginally effective products are just fuel system cleaners that are designed to remove or reduce carbon deposits on engine valves and fuel injector nozzles or in combustion chambers and EGR (Exhaust Gas Recirculation) valves. However, all high-quality automotive fuels sold in the USA already contain similar, if not always identical, detergents and solvents, so buying and adding a can of what is already in the fuel you have in your vehicle’s fuel tank makes little to no practical sense.

So, here’s the thing; the detergents and solvents in pumped fuel have absolutely no effect on the substances that accumulate in catalytic converters in terms of removing them. Therefore, if you do notice a slight improvement in the performance and fuel economy of your vehicle after having added a can (or two) of catalytic converter cleaner to your fuel tank, it is because the added solvents and detergents have removed some carbon deposits from the valves, EGR valve, and/or the combustion chambers. Taken on its own though, this can never be a bad thing because removing or even reducing carbon deposits improves the engine's volumetric efficiency.

Nonetheless, the moral of the story is this: catalytic converters that operate at efficiency levels of 95% or above do not need cleaning because they are working as intended. Moreover, if a catalytic converter does become clogged up with carbonized oil and fuel residues, there is no substance known to modern science that can dissolve and remove these deposits through the exceedingly small channels in the converter’s core.

Sure, some solvents might dissolve carbonized fuel and oil residue at the very (inlet) ends of the channels in the core but in all cases, the exhaust stream will force the partially dissolved carbon further into the core, where the carbon sludge will solidify again. As might be expected, this will render the catalytic converter useless because it will now be completely blocked, as opposed to only being partially restricted.

NOTE: In the interest of fairness, it must be said that catalytic converters on heavy trucks and other heavy-duty applications can be successfully cleaned out and restored. However, this is an entirely different ball game because these catalytic converters are specifically designed to allow for cleaning because of their extremely high replacement costs. In practice, though, cleaning out heavy-duty catalytic converters requires hugely expensive specialized equipment and chemicals that are not available to the light vehicle aftermarket, or the public at large, for that matter.

In the real world, the cat cleaning scam is successful because even light-duty catalytic converters are very expensive, and cans of "miracle" cat cleaners don't cost much. Dishonest mechanics and sketchy repair shops leverage this fact as much as they can, because few car owners can easily scrape up the couple of thousand dollars it takes to replace a modern three-way catalytic converter that operates below its minimum allowable efficiency threshold.

The best advice we can offer when you encounter a shady operator that is offering to restore your catalytic converter(s) is to determine if your converters are still covered by a warranty. Federal law requires that OEM catalytic converters be warranted for at least 8 years or 80,000 miles (whichever comes first), so you might be covered under warranty conditions.

We also recommend that you have your vehicle diagnosed for possible abnormal fuel trims and/or an excessive oil consumption rate before you replace one or more catalytic converters since these issues might void the federally-mandated warranty on the replacement converters, which brings us to-

The refurbished cat scam​

Another prevalent scam involves the proliferation of so-called refurbished/rebuilt catalytic converters. These units always cost a fraction of what an OEM or OEM-equivalent converter costs simply because the so-called rebuilt converters are often used converters salvaged from wrecks in junkyards or worse, converters that were stolen from vehicles parked in streets, parking lots, and even in people's driveways.

All it takes to “rebuild” salvaged or stolen catalytic converter is to clean off and paint the outside casing, which, if done well, makes it very difficult for an average car owner to spot a stolen or ostensibly rebuilt catalytic converter. Moreover, so-called "rebuilt" catalytic converters rarely (if ever) come with a warranty of any kind, which is a dead giveaway that there is something wrong with catalytic converters that typically sell for less than half of what comparable legal converters would sell for.

The problem with so-called rebuilt catalytic converters is that the cores are generally not available to the aftermarket, meaning that there is no way that anybody can open a used converter's casing, replace the old core with a new one, and reseal the casing to meet federal standards. It simply cannot be done because the cores are not available in the aftermarket.

Note, though, that not all cheap catalytic converters are always rebuilt or refurbished. In many cases, new, but cheap converters are imported from China and several studies and investigations have shown that these converters often do not contain enough catalytic metals to convert any component(s) of exhaust gas into anything else. In short, many catalytic converters imported from China and other Asian countries, of which Pakistan is a leading example, are less than useless and should therefore be avoided at all costs.

Again, in the interests of fairness, it must be said that several manufacturers in the USA build bespoke catalytic converters for specialist racing or performance applications. However, in these cases, the high-flow cores are made to special order by the manufacturers of “normal” cores, which means that such specialized products often cost more than an average new passenger vehicle.

In practice, there are no known rebuilders of catalytic converters anywhere in the USA that produce genuinely rebuilt catalytic converters for light vehicles that meet even the minimum standards set by the EPA and other state and/or federal regulatory bodies. As such, it is fair to say that all so-called “rebuilt” catalytic converters are either used or stolen, and while some of these converters might work for a while, the wiser option would be not to buy a possibly stolen converter or just as likely, a used converter that might fail within a few weeks, or even a few days, which leaves us with this-


Catalytic converters are critical components of modern exhaust after-treatment systems, and as such, there are no easy or cheap ways to get around failed or defective converters.

Also, bear in mind that damaged, clogged, or restricted catalytic converters can cause severe damage to vehicles. Typical examples of such damage include, but are not limited to accelerated engine wear due to excessive fuel consumption, damaged and destroyed turbochargers or superchargers, and even catastrophic engine failure due to overheating caused by excessively high exhaust back pressures. Based on this, the only reliable and most cost-effective remedy for catalytic converter efficiency issues is the replacement of damaged converters with OEM or OEM-equivalent parts. And, of course, ensuring that your vehicle is always serviced and maintained to OEM standards and recommendations.