Catalytic Convertors

McNamara's stock and service a very complete range of replacement catalytic convertes both universal and original equipment to suit most models, including Japanese imports, at very competitive rates.  Please contact us with any queries.

The catalytic convertor is fitted into the front part of the exhaust system as close as physically possible to the engine.  it does this with a combination of heat and a precious metal catalyst.  The reduction of emissions is caused by a chemical change known as a thermal reaction.

The Convertor is constructed of a case which houses, a normally ceramic, honeycomb monlith, the internal surfaces of which are coated with a combination of platinum and rhodium.  Endways on this monolith is visually similar to drinking straws, with hundreds of flow channels that allow gases to come into contact with the catalysts material.

From the year 1993, alomost all-European regsitered cars have been fitted with a three way catalytic convertor.  Other types of converter include the two way converter, now superseded due to its lack of ability to reduce nitrous oxides.  The three way converter with air, performs the same as the three way convertor, but has clean air pumped into a mid way point to assist the oxidation process.

Operating efficiently the three way converter reduces reduces nitrous oxide into nitrogen, and as with the two-way converter it oxidizes unburned hydrocarbons and carbon monoxide into water / steam and carbon dioxide.

The catalytic converter operates independently from the engine but it does rely on two factors firstly it relies on the exhaust gases to warm it up, the minimum operating temperature is around 400 degrees centigrade.  Secondly a clean running engine.  Any catalyst can only operate efficiently if the correct amount of oxygen is in the exhaust system.  Which brings us to the point of causes of failure.  The principle causes of failure are:

1) Carbon pollution, leading to a blockage of the catalyst.  Too rich a fuel mixture and / or oil antifreeze entering the exhaust causes this.  Carbon deposits initially restrict the operation of the catalyst by coating the available reactive surface.  If the fault is allowed to continue the actual exhaust flow through the exhaust is restricted, leading to a reduction in engine power and eventaully total blockage.

2) Convertor melt-down.  Usually caused by neat fuel entering the exhaust.  Once this neat fuel enters the hot catalyst it ignites and superheats the catalyst and causes melt-down of the ceramic monlith.  The melted ceramic may block the exhaust and the fault will show as above.  Other signs are the converter glowing red and /or discoloration due to the heat generated on the other surfaces of the converter.

3) Internal fracture of the catalyst.  This is usually caused by external / internal physical damage i.e. something hitting the exhaust, internal damage or the actual monolith coming loose internally.  The most obvious signs of this fault are exhaust rattles / vibrations.

Catalytic converters are genrally agreed to have a service life of between 50 - 100,000 miles but there are several factors, which will reduce this life drastically.  These include:

  • Lead pollution

  • Engine ignition/fueling faults

  • Vehicle use, short journey use prevents the converters from reaching operating temperature.

When Catalytic converters fail prematurely there is usually a reason for it and it is important that this fault is addressed before the replacement is fitted, to prevent the damage that will occur to the new unit.  Converter faults take time to show, so it is possible to replace one and the vehicle will function OK, pass all emissions tests etc, but will still have the underlying fault, which will shorten the replacement service life.

When diagnosing catalytic converter faults before testing for emission ssystem faults, ensure that the engine is up to full operating temperature, a road test is recommended.  One of the common faults that you will come across is a blocked / restricted catalyst which will show up as a low powered unresponsive engine this can often be confirmed by unbolting or dropping the front of the cat and visually examining the monolith.  non starting can also be indicative of a blocked cat.

The ideal method of checking the actual operation of the catalyst is to measure the exhaust gases before and after the converter, with a four gas tester, and measuring the actaul levels of the gas conversion, this is not always practical.  Very few cars are fitted with an access point in the front pipe, enabling accurate measurement.  The use of a four-gas tester is invaluable in diagnosing fueling faults.  For general testing, vehicle specific data needs to be consulted, emission data varies betwen cars.  What is acceptable for one may not be acceptable for another.



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