Catalytic converters are an integral part of a car’s engine management system, not simply a part of the exhaust system.

Catalytic converters are designed to reduce:

• Carbon Monoxide
  (A colourless and odourless poisonous gas)
  
  
• Hydrocarbons or Volatile Organic Compounds
  (A major component of smog, produced mostly from evaporated unburned fuel)
  
  
• Nitrogen Oxides
  (NO and NO2 (combined called NOx) which contribute to smog and acid rain causing irritation to human mucus membranes.)

When gases are emitted from the exhaust chamber, they pass into the exhaust system. A lambda sensor measures and adjusts the air / fuel ratio and stops unburnt fuel reaching the CAT directly.

When the gases reach the CAT they are exposed to a coating of precious metals which act as a catalyst on these gases, converting them from harmful to safe fumes.

Once the temperature in front of the exhaust system reaches 250°C – 270°C, the catalytic converter begins to work. This should occur around three minutes from a cold start.

As a direct result of too rich fuel mixture and/or oil or antifreeze entering the exhaust system, deposits build up on the internal monolith, leading to a partial or complete blockage.

This blockage or partial blockage can have the following effects:

Carbon deposits can reduce the surface area which allows the thermal reaction to take place. If these carbon deposits are allowed to continue over an extended period, eventually all will become blocked and you might notice a reduction in the performance of the vehicle. If the vehicle is used with the monolith blocked the catalyst might become noisy. This is a direct result of the exhaust gases seeking an alternative route of escape and destroying the substrate, which separates the monolith from the stainless steel casing. This renders the catalyst completely inactive.

If neat fuel enters the exhaust system, the fuel will ‘superheat’ the catalyst and the monolith might be partially or totally destroyed. This may block the exhaust or just be burnt and blown out of the system.

A catalyst suffering this type of failure is easily recognised. The converter glows red in operation, or is externally discoloured by the effects of the extreme heat the catalyst has experienced.

• Lack of servicing
• Some catalysts contain ceramic bricks which can break if they suffer a knock
• Engine misfire or bump starting a vehicle
• Leaded fuel can poison the catalyst
• Leakage of welds or pipes can change the air / fuel ratio

Catalysts are exposed to extremes of heat and mechanical stress which can result in damage after a period of service.

Malfunction Indicator Light (MIL) – Illumination of the MIL does not necessarily mean the CAT is faulty. Diagnosis is required prior to a replacement CAT being fitted.

From 2009 it became a legal requirement for all CATS made, sold and installed to have R103 Type Approval and affects all cars and light commercial vehicles up to 3500kg registered on or after 1st January 2001.

The only exclusions are vehicles not originally approved to EU emission standards e.g. grey imports subjected to Single Vehicle Approval.

Existing stocks of non R103 compliant CATs can be sold or fitted to vehicles registered on or before 31st December 2000.