Catalytic Converters

In 1993, it became a legal requirement in the UK for vehicles to be fitted with Catalytic Converters as part of the Euro Emissions standards to improve air quality. Today, we manufacture Europe’s largest range of Catalytic Converters with our range covering:

Alfa Romeo, Audi, Bedford, Bentley, BMW, Chevrolet, Chrysler, Citroen, Dacia, Daewoo, Daihatsu, Dodge, Ferrari, Fiat, Ford, Galloper, Honda, Hyundai, Innocenti, Isuzu, Iveco, Jaguar, Kia, Lada, Lancia, Land Rover, LDV, Lexus, Lotus, Mazda, Mercedes, MG, Mini, Mitsubishi, Morgan, Nissan, Peugeot, Porsche, Proton, Renault, Rover, Saab, Ssangyong, Seat, Skoda, Smart, Subaru, Suzuki, Toyota, TVR, Vauxhall/Opel, Volkswagen, Volvo

What are they and how do they work?

A Catalytic Converter (Cat) is an emissions control device that reduces the level of harmful exhaust gases that enter the atmosphere. It works by converting exhaust emissions into less harmful ones when the gases pass through it, triggering a chemical reaction.

Euro Emissions

The table below contains the European Emissions Standards for passenger cars, defining the acceptable limits for exhaust emissions of new vehicles sold in EU member states. EU Emissions Standards are subject to amendment from time to time, for example, vehicles made after March 2001 (Y-reg onward) are now required under UK law to be fitted with type-approved, homologated Cats. Anything from Euro 3 onward are classed as H-parts and can be identified in the BM range by a “H” suffix. BM Catalysts work tirelessly to keep up-to-date with these standards and develop our parts to meet the ever-changing needs of both the industry and our customers.


Understanding Emissions Reports

A vehicle will fail an emissions test on either, or all, of the following factors:

Carbon Monoxide (CO) Levels     Hydrocarbons (HC – unburned fuel)     Lambda Reading

CO Failure can mean several things, but is most likely caused by a poorly running vehicle. If a Cat becomes contaminated with unburned fuel, it will become unable to effectively convert the gases that pass through it. A new Cat working at full capacity will almost certainly lead to an MOT pass, but unless the fault is rectified, the customer will return the next year complaining of the same problem.

HC Failure is an indicator that the vehicle is over-fuelling. A vehicle will fail on this reading if it has HCs in excess of 200ppm. HCs in excess of 60ppm indicates a vehicle problem that needs rectifying as soon as possible.

Lambda Failure means there is a problem with the air:fuel ratio on the vehicle. If the lambda reading is less than 0.97, the vehicle is running too rich. If the lambda reading is greater than 1.03, the vehicle is running too lean.

What can go wrong? Warranty problems

Catalytic Converters fail for many reasons as mentioned above. In many cases, the failure has little to do with the Cat itself, but instead is the result of another fault within the vehicle. It is important to rectify any problems before a new Cat is fitted. Below is a list of problems related to a vehicle fault, and therefore not covered under a manufacturer’s warranty.

Emissions Failure

When a vehicle fails an MOT emissions test, the blame is usually placed on the Cat. However, the Cat can only convert what gases pass through it; if the vehicle is running poorly, the Cat will become contaminated with unburned fuels and become unable to convert the gasses efficiently.

If the Cat is replaced without fixing the vehicle fault, it is highly likely it will pass the MOT test as the new Cat is working at its full capacity and will mask any fault. A tell-tale sign of a vehicle that has not been repaired is that they will return a year after failing the original MOT having failed the test again.

Engine Management Light/Fault Codes

The Engine Management Light will be triggered on the dashboard if the vehicle’s emissions values fall outside the set parameters. The most common fault code relating to the Cat is the “Cat inefficient” code: P0420. It is important to investigate the following faults before condemning the Cat:

  • Lambda fault
  • Air leak in the exhaust system
  • Retarded spark timing

Different Styles of Cat

Underfloor Cat

This type of Cat is positioned directly on the underside (or underfloor) of the vehicle.

Manifold or Close Coupled Cat

These type of Cats are designed to fit directly on to the vehicle’s manifold where the exhaust gases are hotter. This allows the Cat to reach optimum temperature more quickly and start the gas conversion process a lot faster


This Cat is part of the manifold and is consequently positioned where the exhaust gases are at their hottest. Again, this helps speed up the gas conversion process and helps the Cat avoid any unnecessary build-up of unburned fuels that could impact on the effectiveness of the part and could cause damage to it.