Typically converter failures fall in to one of the following categories:
Two-way converters operate relatively efficiently with a lean fuel mixture. Ineffectiveness in controlling NOx led to the introduction of three-way converters.
Three-way plus air converters were used in vehicle emissions systems in North America during the late '70s and early '80s.
Inside this converter there are two substrates. The front, coated with the precious metal rhodium, is used to reduce NOx emissions into simple N2 and O2. This process is most effective when little O2 is present (rich mixture). That is why it is located upstream of the air tube.
Since a rich mixture is high in HC and CO, an air pump and tube supply additional O2 to this mixture before it enters the second substrate.
The second substrate, coated with the precious metals palladium and platinum, allows oxidation of HC and CO to less harmful emissions CO2 and H2O.
This system was not very efficient and was phased out in the early '80s, when the current three-way converter was introduced.
Three-way converters have been used in vehicle emissions control systems in North America - and many other countries - since 1981.
The three-way without air uses advanced catalyst chemistry to store and release O2, in conjunction with an O2 monitoring and control system.
This system utilizes one or more O2 sensors to oscillate the fuel mixture between lean and rich conditions. This oscillation, combined with the O2 storage and release on the catalyst surface, allows for optimum reduction of all three emissions.
Three-way converters are used in conjunction with OBDII diagnostic systems on today's vehicles. This system alerts the driver when the converter is not working at peak efficiency.
Any malfunction causing an unusually high level of HC or CO (in conjunction with high levels of O2) to enter the converter, will dramatically elevate its temperature.
Potential causes for high HC readings may include:
Catalyst poisoning occurs when the converter is exposed to emissions containing substances that coat the working surfaces, enveloping the catalyst to the point it cannot contact - and treat - the exhaust.
Potential causes for coated or fouled substrate may include:
The primary cause of structural damage is road debris striking the converter. Normally, some evidence of impact is visible on the converter shield.
Other conditions that may cause structural damage: