Search FAQ

  • Technical Information on Catalytic Converters and Particulate Filters

    • What is a Catalytic Converter, and what is a Particulate Filter?

      If the combustion in vehicles were perfect, only water and CO 2 would be produced. However, this is not so and a series of other gases and particles are emitted; some of which are harmful and need to be removed. The main pollutants produced by an internal combustion engine are carbon monoxide (CO), nitrogen oxides (NO x ) and unburned hydrocarbons (HC). In addition, particles are generated, especially in diesel engines, which the law requires to be removed according to EIV standards. The catalyst promotes the conversion of these pollutants into other non-toxic, non-polluting gases, such as water, CO 2 and N 2 ; while the particulate filter retains and removes this matter from diesel vehicles, mainly.

  • How do they work?

    • Catalyst

      In most cases, exhaust gases are circulated through a ceramic substrate with a cell structure, which has a catalytic coating containing elements such as Pt, Pd and Rh. This is where the oxidation and reduction reactions take place to convert the noxious gases. The catalyst needs to reach a certain operating temperature for these reactions to take place, so increasingly they are installed ever closer to the engine. Other components are also added, such as tubes, flanges, flexible hosing and lambda sensor socket for monitoring the catalyst.

    • Diesel particulate filter

      Like the catalyst, the diesel particulate filter consists of a ceramic substrate, with a high degree of porosity. It also has a cell structure, but in this case with the cell inlets or outlets blocked alternately; in other words, when the inlet is blocked in one cell, the output is obstructed in the adjacent cells. This means the exhaust gases have to pass through the porous walls of the filter, where the particles are retained. Depending on the ceramic material used in the manufacture of the substrate, the filters are either cordierite or silicon carbide. The cordierite filters are lighter, simpler to manufacture and cheaper; while silicon carbide filters have better physical features and last longer. The filter becomes clogged if it is not cleaned in any way, and so has to be “regenerated” to remove the particles. This is done by a series of sensors and peripheral components available to the filter, so the vehicle control unit can decide the optimal moment to start the regeneration and control it. The regeneration consists simply of burning off the particles, so that the filter is again able to filter gases properly. As with the catalytic converter, other components are added depending on the particular design, e.g. temperature probe sockets, pressure tubing and lambda sensor sockets for the vehicle's control unit to start and control filter regeneration.

    • Tips for replacing

      There are many reasons to replace a catalyst: blows that rupture the substrate, poor attachment of the substrate due to degradation of the blanket, injection problems, ageing of the catalyst, loss of efficiency or the use of inadequate fuel or oils.

      Not replacing them can also lead to the failure of peripheral components for controlling and regenerating the filter or catalyst, such as lambda probes, temperature probes, the EGR valve, the differential pressure sensor; while other components can lead to the filter being blocked, which may result in a loss of engine power.

      Repeated short-run journeys can also cause problems with the filter if the appropriate temperature is not reached and regeneration is cut off.

      It is especially important to find out the cause of any problem with the filters. If not, the same problems will recur with the replacement filter. Therefore, a diagnostic device must be used to check for the presence of faults in other vehicle components which may affect the proper operation of the filter and catalyst.

      The following procedure is used for the disassembly of catalysts and filters:

      - Apply easing oil to the bolts and nuts.


      - Remove the lambda probes, temperature probes and/or pressure tubing.

      faq02 faq03

      - Release other attachment components, such as brackets and hooks.


      - Loosen and remove screws.


      The following procedure is used for the assembly of catalysts and filters

      - Make sure that the spare corresponds to the make, model, motorisation, power and year of manufacture of your vehicle.

      - Clean the fastening or coupling components (e.g. flanges, rings and manifolds) and make sure there is no debris from joints or rings.

      faq06 faq07

      - Check there is no foreign matter downstream of the component to be replaced.

      - Fit joints and rings and position the new catalyst or filter correctly. Apply the necessary torque to the clamping components to ensure proper fitting.

      faq08 faq09

      - Place and tighten probes, pressure tubing and supports.


      - Start the engine and check there are no leaks in the connecting components.

      - Once the problems with the catalytic converter or filter are resolved, both need to be connected to the Vehicle Management control unit to clear any possible error codes that may have occurred.


      - Following the manufacturer's instructions, the particulate mass in particulate filters may need to be zeroed and/or a forced regeneration performed for the correct operation of the filter.

  • Note

    Usually, a filter or catalyst fails due to problems elsewhere in the vehicle components or systems. Therefore, faults should be properly resolved by visiting a competent workshop. These have the appropriate knowledge, tools and specific electronic equipment required to find out the cause of the failure and ensure the filter or catalyst is properly replaced. If not, replacing the parts will not solve the problem and the same problems will recur in a short time.