Catalytic converters were originally developed for factory smokestacks to combat smog in Los Angeles early last century by the Frenchman Eugène Jules Houdry. He patented the first automotive catalytic converter in 1956, which was technically a two-way catalytic converter. A catalyst is a substance that participates in a chemical reaction but is not altered or modified during that reaction.
Two-way oxidation catalytic converters use the precious metals platinum and palladium as catalysts. Two-way cats were originally installed on US vehicles starting in 1975, but they had no provision to control oxides of nitrogen (NOx), one of the two key compounds that cause acid rain. In 1981, the law changed to require three-way catalytic converters which also contained rhodium as a catalyst to control NOx. In many sources, the term three-way catalytic converter is abbreviated as TWC.
For rhodium to do its job, precise fuel mixture control with a rich / lean cycle is needed. An oxygen sensor is employed to control the mixture. The sine wave of the oxygen sensor output causes a rich / lean cycle. When the fuel mixture is lean, excess oxygen is captured by the platinum and palladium to convert carbon monoxide (CO) and hydrocarbons (HC) into carbon dioxide (CO²) and water (H²O). Then during the rich cycle when minimal oxygen is available, the rhodium uses the CO², or CO, or H² to liberate and capture the oxygen from NOx to form N², which is the normal form of nitrogen found as about 80% of air.
Diesel exhaust has a typical high level of oxygen (O²) which precludes the use of rhodium to remove the oxygen from NOx. In diesels built before 2007.5, there really is no way to accomplish the rich / lean cycle with diesel injection to lower the oxygen levels, so those diesels can only use a two-way oxidation catalytic converter. Diesels have their own version aptly called the Diesel Oxidation Catalyst (DOC) which includes aluminum with the platinum and palladium of the original two-way catalytic converter.
As of the 2007.5 model year, which corresponds to the introduction of the Common Rail diesel injection system (CR), all diesels were required to have a Diesel Particulate Filter (DPF) in addition to the oxidation catalyst plus two new converters for NOx and SOx (oxides of sulfur). The largest reduction in NOx in the common rail diesel system is in the diesel particulate filter (DPF). The DPF collects soot, which is mostly carbon, and burns it into ash. The process is called Regeneration. In general, burning almost anything involves oxygen and oxidizing. During regeneration, oxygen molecules are torn off of the NOx molecules and are used to burn the soot into ash.
Lastly, NOx is controlled by an additional converter called an SCR converter, meaning Selective Catalytic Reduction. Using urea in a liquid called AdBlue, ammonia is generated inside the SCR. which causes the NOx to be reduced to nitrogen (N²) and water (H²O). The generic term for this mixture is Diesel Exhaust Fluid, or DEF. Good advice for customers who want to shop for cheap DEF instead of the Factory AdBlue version of DEF: Make really sure the label states that it passed the ISO 22241 standard. Be aware that aftermarket DEF could be diluted, contaminated or degraded from age. DEF is a bit like coolant in that it should not allow rust in the system. Bad DEF can corrode the heater and injector.
Another converter is used in Common Rail diesels for SOx reduction. The biggest reduction in sulfur occurred in two stages when EPA mandates caused sulfur to be removed from diesel fuel. Diesel fuel from last century used to contain up to 5000 parts per million (ppm) of sulfur, primarily as a lubricant. Bunker fuel for ships could be as high as 50,000 ppm sulfur. Near the end of the 1990’s, highway use diesel changed to 500 ppm sulfur, and many diesel fuel pumps started leaking. Their O rings and gaskets, although made from viton, could not take the shock of the change in volatile esters, which are the aromatic compounds in fuels, oils and coolants. In 2006, the EPA mandated ultra-low sulfur diesel (ULSD) which lowered the amount of sulfur to 15 ppm. Again, another rash of leaking fuel pumps was the price for cleaner air. The SOx converter is the final solution.
Both the NOx and SOx converters experience an occasional complicated regeneration cycle involving extremely rapid and accurate mixture changes. These converters both appear to be trouble free for the long term.
P0420 Code: Catalyst System Efficiency Below Threshold
It would seem a fairly common malady for vehicles 10 years old and beyond to have a check engine light come on for the P0420 DTC (Diagnostic Trouble Code), indicating a possible catalytic converter problem. To understand how the code is generated, one must realize that the second oxygen sensor after the catalytic converter (B1S2) has a voltage output commensurate with the mixture after the catalytic converter has cleaned up the exhaust. A steady voltage of around half a volt while idling indicates a good catalytic converter reaction, whereas a voltage over about .65 volts and/or a varying voltage indicates a poor catalytic converter reaction. That failure level voltage is not written in stone. The engine ECU monitors the changes and relationship of the front oxygen sensor signal and the rear oxygen sensor voltage, approximates the current cat temperature, and does some complicated math using rpm, load and temperature signals to sort out when to generate the P0420 code.
There is more than one way to scan a cat. One can also check the temperature difference between the front and rear of the catalytic converter. A difference of about 100 °F generally indicates that the converter is working, but may not indicate high efficiency, just that there is a reaction taking place. The external cat temperature varies wildly while driving, and the actual core stays cooler with the best mixture control. Expect outer temperatures between 300 °F and 500 °F while checking, the core itself is going to be way hotter. Normal cat operating temperatures while driving will be at least 500-600 °F, and can sky rocket over 1000 °F to 1600 °F at full tilt boogie up a hill.
The P0420 Catalyst System Efficiency Below Threshold code can be caused by a catalytic converter rendered inefficient through damage and/or contamination. That same code can be caused by other fuel system and/or engine maladies such as vacuum leaks, exhaust leaks, old or failed oxygen sensors, retarded spark timing, misfires from ignition component or injector failures; mixture problems caused by MAF or MAP sensors, coolant or air temperature sensors, and lastly contaminated fuel or oil. It is possible to have an old cat pushed over the edge to P0420 code failure due to one of those said maladies that isn’t even bad enough to generate its own Diagnostic Trouble Code.
The ceramic core of a catalytic converter is quite smooth, so it is coated with a substrate that makes it bumpy to greatly increase its surface area. That substrate contains the precious metal catalysts. Catalytic converter failure is always some combination of contamination and damage of the substrate and/or core. Since exhaust contains burned engine crankcase vapors pulled in through the PCV system, the molecules of contaminants and certain additives in engine oil can coat the catalytic converter substrate and diminish its efficacy. The heat generated in the catalytic converter during misfires can sinter the substrate. To “sinter” means to slightly melt or fuse, which generally corresponds to melting the bumpy substrate more flat, reducing its catalytic ability. Extreme overheating can cause the cat core to break apart, at least getting loose and clanging around, and maybe shedding cat gravel into the exhaust system.
Some models have both a pre cat and a main cat, but the second oxygen sensor is after the pre cat, and does not measure the efficiency of the main catalytic converter. Pictured is the front section of the exhaust of an Audi 2.8 V6, notorious for the P0420 code. The tailpipe emissions can be very clean and still have failure codes for cats on one or both sides.
So the big question is can one clean a catalytic converter? Or better said, can the P0420 DTC be repaired without cat replacement? The answer is complicated, but with a simple truth. Contamination may be somewhat cleaned, damage is irreversible. First fix any other issues that may be contributing to the code. Next, one can try cleaners such as Cataclean by Mr. Gasket, The German OXICAT – Oxygen Sensor & Catalytic Converter Cleaner, or BG Chemical’s BG 44K® Fuel System Cleaner.
The best cure of course is prevention. Never allow misfires. Change ignition components on time. Repair problems when they first appear. Prevent cat contamination. Perform frequent oil changes with quality oil. Don’t use screwy additives in the fuel or oil. Use high quality fuel with cleansers and stabilizers, not the cheap crap. Do not allow the extreme buildup of carbon until the engine misfires or runs rich. Age is a definite factor, but almost any properly maintained engine can get 20 years or more of lifespan from a catalytic converter, depending on the model and the pilot.