Catalytic Converters

Catalytic converters are part of your car’s exhaust system which converts noxious gases from the car exhaust into Carbon Dioxide, Nitrogen and Water.

Frequently asked Questions (FAQ)

Q: Do I need to change my cat?

A catalytic converter can be an expensive item, so you don’t want to change it unless it is absolutely necessary. .

Q: How do I know if my Cat is working?

A: Only in extreme cases of cat meltdown will a total blockage occur, you will know that something has gone wrong because the car will not run! But mostly, you will only find out when the car fails the emission test at the annual MoT.


It is important for owners of vehicles fitted with catalytic converters to understand that the catalyst is only one in a number of components, which collectively make up the emissions control system. Experience has shown that there are a number of reasons why vehicles fail the emissions test and, in many cases, defects which lead to excessively high emissions are often rectified by a simple adjustment. These defects can usually be rectified at little cost to the owner.

Where replacement parts are necessary, costs can increase significantly and it is important, therefore, that defects are accurately diagnosed. Incorrect diagnosis may lead to high cost items (such as fuel injection pumps, fuel injectors, catalytic converters, etc) being replaced unnecessarily. For day-to-day use of the vehicle, and care of the emissions control system, owners are advised to consult the vehicle handbook or seek advice from the manufacturer?s agent/dealer.

Accurate diagnosis of engine management systems often requires the use of diagnostic equipment and appropriately trained vehicle technicians. Motorists should, therefore, seek advice from reputable vehicle repairers before agreeing to repairs

How CATs work

If your car runs on petrol and was built after 1992, it will have a Cat (catalytic converter). It may even have one if dates from before 1992.

Environmental lobby pressure on Governments has forced them to legislate to attempt to limit potentially damaging emissions. The Cat is a device located in the exhaust system which converts unwelcome gases from the exhaust into harmless Carbon Dioxide - the same as we breathe out - Nitrogen, which forms 80% of air, and water vapour in the form of steam. It does this by passing the gases over a fine honeycomb (usually of metal) that is plated with precious metals such as platinum, palladium and rhodium. It is these metals that start the chemical reactions.

How your CAT is Tested

There are several ways of testing your Cat. Make sure that your garage has invested in the latest technology so that you can be confident that the engineering report is accurate: • Emissions test. As mentioned above, by measuring the car’s emissions it is possible to tell from the readings that the cat has failed.

• Rattle test. If the ’honeycomb’ inside has broken up, when the Cat is pounded with a fist, the rattling from inside gives the game away. (Don’t try this yourself unless you are certain the Cat is cool enough to touch).

• Temperature. Using a specialised laser pyrometer (a thermometer used for measuring very hot temperatures), spot check the inlet and outlet temperatures. The rear of a functioning Cat is much hotter than the front.

• Back pressure. Measure the exhaust back pressure to see if there are any blockages.
Reasons for CAT failure

All too often a garage will sell a new Cat to a customer without checking the underlying reason for the failure.You need to have an expert opinion on the reason for the failure by a reputable garage who have the equiptment to do a thorough check of all the exhaust components.
• Wrong Fuel. Put simply, leaded fuel wrecks the Cat. It will soon get blocked and stop working.

• Faulty Oxygen (Lambda) Sensor. This device sits in the exhaust pipe and measures the amount of oxygen in the gases. If it is defective, in time, an over-rich mixture (too much fuel) will poison the Cat.

• Misfire. These are usually caused by a failure in the ignition system. Fouled spark plugs, broken-down ignition leads or cracked distributor caps cause the engine to ’stutter’, this is usually the beginning of the cat’s destruction. Every time the misfire happens, some unburnt petrol finds its way down the exhaust pipe. This fuel then poisons the cat as above.

• Fouled Injectors. Injectors spray finely controlled amounts of fuel into the engine in response to the demands of the driver. Certain inferior brands of fuel lack detergent and in time cause deposits to form on the injector nozzle. This prevents it from closing properly and can allow unwanted petrol to leak into the engine. This will eventually reach the Cat and poison it.

• Failed MAP sensor. The Manifold Air Pressure Sensor is one of the crucial engine management sensors; if it fails it will cause a rich mixture.

• Bump Starting the Car. If you attempt to bump start a car (with a run down battery) this has the same effect as a misfire. Unburnt fuel travels into the CAT and destroys the catalyst.
Fuel & Exhaust
Emissions - Petrol
Emissions - Diesel
Diesel Particulate Filters

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