The following information is intended to explain the function of a catalyst, lambda sensors and OBDII on-board diagnostics. It is a guide only and as such, we recommend that a catalyst should only be fitted by a competent mechanic with some knowledge of catalytic converters. After reading this information page it will be clear the catalysts are very reliable and will only fail under extreme conditions or when past normal operating life!

Function of a catalytic converter  As exhaust gases pass through the catalyst substrate, hydrocarbons, carbon monoxides and nitrous oxides (harmful pollutants) are converted into water, carbon dioxide and nitrogen. It is estimated that there is a conversion rate of approximately 90%.

How does a catalyst remove harmful gases?  Inside the catalytic converter is either a ceramic or metallic honeycomb structure (about the size of a football pitch) which is coated with a combination of precious metals (platinum, palladium and rhodium). At particular operating temperatures, these metals react with the exhaust gas by removing oxygen molecules. The change in molecular structure of the pollutants converts them into non-harmful gases.

How does a catalyst combine with the engine management system?  It is essential that a catalyst works to its maximum potential or stoichiometric point. Therefore there needs to be a feedback mechanism between the catalyst and ECU to make sure this happens.
On most cars, a lamdba sensor (or Hot Oxygen Sensor) is positioned upstream of the catalyst to measure the amount of oxygen in the exhaust gas. The sensor develops a voltage of between 0.1V and 0.9V. A low voltage is measured in lean conditions (where the gas is high in oxygen and low in fuel) and a high voltage in rich conditions (where the gas is low in oxygen and high in fuel). As the exhaust gas changes from rich to lean and vice versa this is called the switch point. As this point is passed the sensor tells the fuel injection system to increase or decrease fuel.

OBDII On-Board Diagnostics  Exhausts that use only 1 lamdba sensor do not provide any way of determining the condition of the catalyst itself. Therefore, a system was introduced called OBDII whereby a sensor is positioned upstream and downstream of the catalyst. The downstream sensor calculates the conversion efficiency of the upstream sensor by recording the ratio of switch rates of both sensors. The ratio should be below 75%. Above this and an error code is returned via the ECU to the dashboard indicating that the catalyst is under-performing.

Use 200cpsi for an engine using OBDII On-Board Diagnostics  If a car has OBDII, it is necessary to use a catalyst that will convert the right percentage of harmful gases. Therefore, if a sports catalayst is used, you should use a 200cpsi substrate. This has more surface area and therefore more conversion capacity. You can see the difference between 100cpsi and 200cpsi below 

Select a sports catalyst: 

To choose the correct catalyst for your car click here.